You will happily take payment for postage whilst advertising that collection is available. Then, when you go to book the collection, "Sorry, collections cannot be arranged right now. This service will be available again shortly, so please try again later.". This has happened a few times - possibly it is unscheduled/unannounced maintenace at the worst possible time which is just before the midnight cutoff. Or possibly the webservice is down from time to time. Either way, you must not advertise collection service on the payment processing page when the API is down. You should allow payment to be taken, but replace the advertisement with a warning that collections cannot be booked. This will allow users to consider sending with another courier, rather than having to go through the unnecessarily long and difficult refund process.

It's like taking payment for a pizza when you know there is no dough, but "it's fine" because you can get a refund in a week's time or you can postpone your party until tomorrow.