There I was being all smug, but it turns out the Paypal-ness of this whole endeavour is a little more complicated than I thought. What I hadn't realised was:
Most people who'll want to buy these cards live locally and therefore there is no need for the Paypal link to have Postage Costs automatically attached to each sale (how boring is this?). Clearly, I've been able to drop off most orders by foot and therefore people need only pay at precisely £3 per pack of 6 cards. And yet....
Some other people might need Postage costs to be added to their order as we don't just want to limit our sales to the SE10 area. Therefore, maybe there should nonetheless be an option to pay for postage? To this end - and rather inelegantly, I'm afraid - I've implemented a nasty little pull down menu you first have to navigate before you can go to the Paypal Checkout area. Essentially, the two options you're choosing from are:
a) You live locally and if you tell us your address we can drop off the cards by foot - or you can arrange to collect from us, whatever's most convenient. (We live in SE10).
b) You don't live locally and therefore you're happy to pay £3.30 per pack because the extra 30p per pack will cover postage.
(I imagine that 30p per pack won't quite cover postage, but I'm happy to take a tiny hit on what is after all a charity endeavour).
I hope this all makes sense now.