You don’t have to be a partner to sell your add-ins.
You can become a Research Associate when you have zero users, which gets you a few SW licenses and which gives you a bit of time to get paying customers. Once you can show two references from paying customers, you can become a Solution partner and at five, you can become a Gold Partner.
But don’t get your hopes up, becoming a Gold Partner has about 100 pages of requirements, mostly regarding the UI. You also need personnel to become one, you need two certified users within your company.
I have two products with the Solution Partner status and I’ve put becoming a gold partner off for somewhere in the far future.
There are no monetary benefits to any level, but you get a page on their site and you can show off the partner product badge. That lets people trust you more, hopefully. As a gold partner, you also get reseller contacts.