@matigo You don't need to use Stripe's SDK, you know, you can just call the APIs yourself. For PrivateLog I'll be using WebPay [webpay.jp]. Reasonable rates (cheaper than Stripe, which is actually really expensive), and the API isn't terrible. I've used it on other projects, directly accessing the API with no problems.
Maybe when I get some scale, I'll go with GMO PG, but right now it's not worth it (both development time cost and actual service cost) because that API is very hairy.