I'm afraid there seems to be some problems with the online license purchase system for Mercury.
The first (technical) one is that the PayPal page didn't at all load in Firefox, I had to start again using MSIE. The second (legal) one is that the receipts received from PayPal aren't acceptable business documents for the transaction. The full name and address of the seller isn't included, nor the name of the buyer, nor a specification of what kind of license has been purchased.
/Rolf
Update:
There is an email sent out at a later time (in this case 2 days after the payment) with a download link, that gives the opportunity to print an invoice. This does presumably take care of the legal problem.