At a mail shop I was told I could not send a letter weighing over about 20 grams without going through a registration process.
When I asked for details, they gave me contradictory information.
Has anyone come across this, and could give me further information.

ctt.pt ?
https://www.ctt.pt/particulares/envi...o-estrangeiro/

I followed the link, but the website wasn't working well.
I eventually found a chat service that said:

To send letters to the USA through CTT, only documents of a personal nature (e.g., letters, manuscripts, drawings) without commercial value are allowed. Sending goods, including gifts, is currently suspended for all services. Addresses must be written in uppercase, with "UNITED STATES OF AMERICA" on the last line. Letters can weigh up to 2 kg, depending on the service. Delivery times are approximately 5 business days for Correio Azul and 7 for Correio Normal, excluding customs delays. For more details, visit https://www.ctt.pt/particulares/envi...dos-da-america.

This is quite different from what I was told.

Check DHL, they are very reliable and maybe not too far from ctt price.

I've seen a number of complaints against DHL, though IIRC these were all about receiving letters/packages.

The situation is currently a mess and very fluid, thanks to the random rantings and idiotic decisions of the US Clown-in-Chief and his mindless minions. I think it was the start of September (2025) when many foreign postal services and the international courier businesses decided to unilaterally stop sending parcels (anything other than paper documents) to the US as a result of changes to the import duty assessment and collection protocols and a lack of clarity as to how they would work.

Rather than risk shipping items to the US that might spend weeks or months awaiting US customs clearance, and potentially stored in the mail/ courriers' warehouses, taking up space (which they might not have to spare, as there has never been an expectation of large-scale storage of inward mail awaiting customs clearance), and then perhaps have to administer the collection of paltry amounts of duty on each individual parcel, mail and courriers outside the US simply stopped accepting tangible objects of value to send to the US.

So here we are.

The restrictions I encountered went far beyond those caused by the US.
The CTT details posted above say you can send letters up to 2kg. But I wasn't allowed to send a letter of just over 20 grams.
That it less than two sheet of A4 paper.

When you said a mail shop in the first post, do you mean an actual post office, or one of those places that acts as an agent for them?

Have you managed to get it sent yet? Have you tried using a different post office?

It was not a CTT shop, so an agent as you named it.

|I didn't give specific details before, as I didn't want to criticise the shop if they were following guidelines.
What happened was very unusual, but I've brought the issue up now because I found that the letter was never delivered.

It was in early August, and they would not send the letter unless I cut out blank sections of the two pages in the letter to bring the weight below 20 grams.
I didn't use any special service, as I was told that individual letters are not signed for, and so the extra services are thought to be of no real value.
I wanted to take a photo of the envelope as a reminder (there would be a time stamp on the photo), but they would not let me do that.
Though there is no evidence that they were responsible for the non-delivery, their other behaviour leaves me with a poor view of that shop.

And it was CTT stamps you bought, was it?

I only ask because I was on holiday elsewhere in the country last month and sent a postcard to the UK. I was offered a stamp in the shop where I bought the card and just assumed it would be a CTT service, so I said yes - but it wasn't. It was some completely other private outfit, with a dedicated postbox in the shop. I didn't twig until after I'd written the postcard and put the stamp on it, when I realised it had another mail service name on it.

I had assumed it was CTT stamps, but they wouldn't let me look at the letter after I had paid.

I have now had a look at the receipt, and the first line says:

CTT - Correios de Portugal, SA - Soc.

so that looks like it was definitely a CTT service.

Yes, definitely CTT.

I did use a small agent-type place once or twice in the past, for convenience but they didn't even have a scale to weigh items on so I thought it best to revert to proper post offices.

It certainly looks like yours got it completely wrong on this. I also disagree with what you were told about special services - for communications to official bodies or other important items, I always use registered post and if it's going to the UK I can track its progress through to delivery. I believe it's also more secure than normal mail.

What I heard about "signed for" services, is that large organisations receive far too many packages to sign for each one individually, and so they "sign" for a sackload. So you cannot prove that the item was received.
Does the tracking facility actually give a benefit other than a feel good feeling that it's actually making progress towards its destination?
The only time I followed a package, it left me frustrated.
It was coming from the States, and quite rapidly reached Porto.
It then seemed to stall, go somewhere else unspecified in Portugal, and then back to Porto.
I never did receive the package.
The sender resent a duplicate (it was a card) after a short period.

I found that unregistered regular mail is faster and more reliable.

Last year I sent a letter to the US (from a small rural post office to a small rural post office) that arrived in 2 days.

Inside EU, regular mail I sent arrived in 4-5 days, registered mail 8-10 days (or never).

Twice, I paid extra to get a returned receipt signed by the addressee, but never got one.

In my experience, it's been useful to know that the item got through the front door and when, at the least.

Otherwise, particularly if you're dealing with somerhing official, you have no idea whether or not it got to its destination. Also - as I understand it and as mentioned above - it is supposed to be more secure.

For normal post - letter or card to a relative or friend - I wouldn't bother.