Why do you think that? People definitely pay taxes by card.
But regardless this contract is _not_ for HMRC payments, its for gov.uk pay which is basically a centralised service that other services can use.
Most underpaid taxes are collected via a change in tax code for the following year.
GOV.UK pay has done £9.2B in payments in the last decade, with an average of £67
https://www.payments.service.gov.uk/performance/
A very crude calculation for £1B a year in payments (thats probably too low) would mean a payment to Ayden (contract is upto £25M over 3 years) of 0.8%