Twenty percent of every sale you send, on all four instruments and the Collection. No quotas, no exclusivity, and nothing owed if it never works out. Gumroad tracks and pays it, so I am not in the middle of it.
I read these by hand rather than handing out an open link, which is why the form asks real questions. It takes about three minutes. The terms are here and they are short.
Being straight with you saves us both the time. These are small, specific instruments, and audience size matters far less than whether your people actually make this kind of music.