Unfortunately I find that when sponsors start up the whole "invite only" and "send us all these docs and screen shots of your sales" are often the ones to look out for. Consider that it's showing that the sponsor views them allowing you to be an affiliate as some sort of favor to you. Once that happens it typically means a lot of other crap is right around the corner. It's just the same for employees in an organization. If the business starts to view you as a liability or cost rather than an asset, you're better off getting away from them. So why should I become an affiliate with someone prone to viewing me as such right from the start?
I take checks. I prefer 50/50 partnerships. I rarely contact a sponsor for things. I send sign ups. If a sponsor views me as a liability then I'll gladly use another because I'd say they are idiots.