The FairPay component is more of a demo than an actual component.  It shows how to use the
`Paypal.svelte` and `BTCPay.svelte` components.  The `FairPay.svelte` implements a "pay what you
want" style of payments.

Why the $10 increments?  There's a form of credit card crime called "carding" where people with
stolen numbers test their cards on any purchase that is $1.  They'll get large numbers of possibly
valid credit cards and they need to test them.  The best way to test them is to attempt a very small
purchase, and if the purchase works then they mark the card valid.

If you allow people to pay any amount as a donation then carders will raid your purchase form and
rack up $1 fraudulent charges.  That may not seem to bad, but your payment processor will then
charge _you_ $15 or more to handle the charge back.  Even if you offer to refund the purchase and
they do no work.  This means if you receive $100 of fraudulent charges you would owe $1500 in fees.

The easiest way to prevent this is to simply only allow increments of $10.  $10 is too high for most
carder testers, so this will deter them (at least until everyone does this).