@ed: “It depends how you pay”
I’ve only ever visited Patreon (and Naked Experiment, until this morning) from a desktop computer.
But I checked the billing on Patreon — it’s treating the £3.50 as a vat-exclusive price (which I believe is not allowed in the UK, prices quoted to UK end users must always include all taxes, but VAT exclusive might be legal and normal wherever Patreon is based). So Patreon is adding 70 pence for VAT and hence the £4.20 billing.