In this scenario we did have an account manager at Stripe, however fortunately and unfortunately the show brought in somewhere in the region of 7X the initial target.

Servereless stampeding a payment processor like Stripe as opposed to something like RDS are two different kettles of fish, as am sure you can imagine, stripe have very high capacity and there is 1:1 relationship to the user unlike DB writes, where there may be many per transaction / action.

I have don't it both ways in the past, queueing payments and processing later. Problem is with doing that you cannot immediately return card errors, which has a large effect on conversion. With something like Amazon it is fine to do that, as users inevitably come back to buy the products they want, for donations not so much.