Gosh, apologies - I've just double-checked and I did use PayPal. I have a credit card saved on my PayPal account. Used that without any issues. I chose to pay in EUR not GBP on the PayPal screen, because then the exchange rate is set by my credit card provider and not PayPal (who tend to fleece you).
I fitted it yesterday, so haven't had chance to sleep in it yet, but it felt pretty good. One thing that I didn't see on the site is that the default package does in fact have two different heights of springs. From the (Google-translated) installation instructions:
Bar 1: 7 flat springs
Bar 2 & 3: a flat spring on each side, 5 high springs in the middle;
Bars 4 to 7: 7 high springs;
Bar 8: 7 flat springs;
Bar 8 is at the front of the van, i.e. where your head is, so it's a bit weird to have your head lower than your shoulders. I've not had a pillow in there yet, though, so full assessment yet to be carried out.
The springs are very easy to remove from the frame, so it would be possible to swap some around. However, it's not clear whether the default configuration is designed for comfort in the middle of the mattress and less at the edges, or simply to do with roof clearance over the width/length of the Cali.