Don't know if this is still an issue, but if any potential buyers are okay with paying a bit extra, here's how you can ensure you send at least X amount to @melancholy-marionette. First, convert how much you want to pay in GBP into your native currency. Right now, the exchange rate is 1 GBP = 1.34 USD so if I want to pay 5 GBP, I have to pay $6.70 USD.
To cover the 30% tax cut, remember that 100%-30% is 70%. You will want to divide the payment amount by .7 or multiply by ~1.43 (NOT 1.3). So if I divide 6.70 by .7, I get ~9.57.
So to pay @melancholy-marionette 5 GBP, I'll need to send $9.57 (9.58 to be safe).
EDIT: I forgot about the PayPal fee. So that's 3.5% plus .39 GBP.
So, divide 9.58 (which I already sent) by (1-.035) and we get ~9.93 USD. Converting .39 GBP to USD comes out to ~$0.52 USD. So I should have sent $10.45. This doesn't account for the itch.io fee which varies by the developer so I'm not even going to try.