Our son needs to renew his UK passport that lapsed during Covid. He has a bank acct in the UK, but the debit card is expired. He is a secondary card holder on our CC accounts. We haven't gone online to do this yet, but a few questions. If there is a hiccup can one save and return to the application later or does it have to happen all in one go? Does the method of payment have to be in the name of the applicant? I worry that as a secondary card holder the payment may be flagged by our bank given it is an international transaction so would want to give our particulars vs his.

Re shipping--we do not have a DHL office locally. We do have UPS. Anyone use them for renewal? DHL will send a driver out to pick up a shipment but they won't even give you a routing number or confirmation of any kind which is too much of a nail biter for me.

Many thanks.

I don't think HMPO cares who pays for the Passport application - so I can't see any issues there.

I used UPS to ship my old passport back to Hemel Hempstead last January, it got there in 48 hours and I had my shiny new Passport in less than a week (HMPO used DHL to ship the new and old one back, separate parcels).

You can use whatever method you choose and are comfortable with to send to the Passport Office. One time I used USPS, another FedEx. Both worked fine, although USPS was much slower.
As VeryVeritas says, the Passport Office uses DHL to send the new passport to you. They deliver to your home. You do not need a DHL office nearby.

I mailed mine in from the local post office. They give you all the documentation for confirmation, routing number etc. Much easier and more straightforward than using a commercial shipping company.