This comes from the Payment Card Industry, and their idea that you should "Know Your Customer," and US Treasury Department, which has zero tolerance for payment processors who take a cavalier attitude towards money laundering, and they go directly after the owners of the company, and press criminal charges.
So while you're selling 100 a month, you're not really on anybody's radar, but start processing in earnest, and they will want all of the info that a traditional merchant account process requires.
Believe me when I say that "Instant Approval" ruffled a lot of feathers with more traditional processors who were forced into long, involved approval processes by PCI.
So you can start immediately at an instant approval processor, but they'll want all the same into before you can get your money. If you search for horror stories, you'll find lots of merchants who signed up for these services who didn't read the fine print about their money being tied up, and complain loudly that the processor should have told them something that important.
You can get out in front of the process with these processors by talking to them. let them knwo that you're about to launch, or offer discounts or whatever, so your spike in volume doesn't look suspicious. That the in-depth approval process as soon as you can. They want to be your partner, but there are rules and laws that they're not going to bend or break for you.
BONUS: one of the reasons that credit cards don't like to be used for crowdsourced funding sites is that there is no guarantee that a good or service will change hands, and therefore they have a harder time telling legit crowdfunding from money laundering "oops, sorry guys, or idea flopped. thanks for the $200 thousand dollars!"