Reading the letter in full, it is clear that when he says he expects and hopes to pay more taxes, it is because he expects and hopes that profits will increase.
But doesn't that go both ways? It's a bad path if you are only allowed to post your thoughts on Twitter if you put forward exactly the right (according to you) political or social views publicly.
The owner of a GPL project doesn't own the copyright for contributed code (and therefore can't license it themselves under GPL), unless they receive a copyright assignment.
For card-present goods and services it's likely lower, but for card-not-present online software sales from a small company, rates with your own merchant account can be even higher than 8%. This is a big advantage of using Stripe and the like.
Credit card rates with your own merchant account are 3% - 8%, depending on your industry and chargeback history.
Federal Marginal tax rates are 10% - 38%, plus maybe 5% in MN.
If they are giving you a 5% - 10% discount for cash, sure, it's probably because they appreciate skipping the credit card. If they give you a larger discount for paying in cash, they probably aren't reporting the income.