How do you declare sales in your country?

Don’t know exactly what you mean, do you mean how to report things officially and pay taxes on the revenue?

I mean how one is reporting an income for tax purposes in own country in practice as there is simply no single proper answer on how to do it .
If you are based in EU and not in Lithuania, for example, you have to deal with VAT even with very low sales.

Don’t know how it works in other country's but here in Belgium (EU country) there is a separate category for royalty's and neighboring rights on the tax reform.

We can report the royalty’s as a natural person separately from other activities we might be doing as a regular employer or as a freelancer who normally sends a bill and invoice to someone.

We actually pay less tax on royalty’s (15%), nice thing about it is that it works with a flat fee for expenses which is up to 50% of what you report (at max up to 35K a year). We only pay tax on the other half. We can go up to 65K a year with this type of arrangement, everything above it should be reported separately.

I assume in other country’s there is some similar arrangements for royalty’s?