I see that a SilverBullet+ license is required for profecssional use. That's fine, because it means it can be a valid business expense. Is there a way to get a normal VAT invoice for that? It's not obvious before the purchase, and it's a requirement for me.
Also, what is in one license? There is a mention of "seat", what happens when the app is installed again? Dual-boot computer? Multiple machines for the same person? Thanks for the info 
Yes, after you make you purchase you can get into the Paddle license manager by clicking the "Manage" button on your SB+ account page. From there, you can go to Payment (left menu) > and then "View Invoice". To update data on the the invoice (e.g. with company data, Tax ID etc.) click the "Update Invoice" button, then "View Invoice" again. This should work.
First of all, this is all honor code so little about this enforced. The intention is that one "seat" equals one user, regardless of how many systems they install it on.
The idea of being able to buy multiple seats at once is primarily for people who want to bulk buy for their team or company. If you buy 10 seats, you will get 10 license keys (which you can name) and distribute to team mates to use to making purchasing and management easier.
Also added this to Frequently Asked Questions — SilverBullet+ now