Hi Kirsten,
To answer your question, I shall take off my Warden's hat for a moment and put on my hat as bookkeeper of Holy Trinity Church Bradford on Avon. Holy Trinity uses SumUp for their contactless payments. They've got a Payaz machine at the entrance for people wishing to donate by card (currently not there, as it's been sent off to be repaired or replaced). They had to splash out over 300 pounds for the gear and are charged 1.1 per cent of every transaction (which is probably some kind of charity rate). Payments received through the machine - minus the transaction fee - are made on a daily basis straight into the church's bank account. The Payaz machine has a gift aid feature where people can enter all the info required to claim gift aid on their donation - which then goes to a 'Give a Little' spreadsheet that can be used to do the gift aid claim to HMRC.
SumUp comes in all kinds of flavours - there are cheaper options with much smaller card readers, often used by small traders (for example at our weekly market here in Bradford on Avon).
As for 'tagging' transactions, the Holy Trinity gear doesn't do this. In fact, I'm just about to correct the coding of a few hundred pounds from 'donations' to 'Street Market' - luckily the person in charge of the machine at the annual Street Market event kept a tab on when the machine was used to sell refreshments etc.
Best wishes,
Klaus Huber
Resident Warden, Bradford on Avon (and, 3 hours a week, Bookkeeper of Holy Trinity Church Bradford on Avon)