updown.io
To accept payments on updown.io, we first added support for Stripe which is by far the most popular payment gateway for startups and for a good reason. Their service is of awesome quality: the UI is gorgeous, the integration is easy, the documentation is great, the API is super stable and well thought. I can't recommend it enough.
We then added support for PayPal which is pretty popular for people who have money on it and don't know where to spend it (it can make it feel like you're spending less when it comes from PayPal wallet), or for people who prefer not to enter a credit card on a new website. This was pretty well received and we're currently receiving about 25% of our purchases from PayPal. The documentation and integration is much more painful than with Stripe IMO, I can't recommend them for that, but not having it is basically dodging potential sales.
Finally we more recently added support of BitPay for #Bitcoin and BitcoinCash payments, which was a pretty easy process but not worth the time in the end due to the low usage and the always changing conditions of the network: the transaction fees got huge after price raise and bitcoin because unusable for small payments, they then introduced support for BCH and a newer Bitcoin protocol for lower fees, but then you need a special wallet to pay and in the end it's too cumbersome, even for bitcoin users, to pay with it. I think unless you expect a bit number of payments using cryptocurrencies it's not worth implementing this solution, and better to accept them manually.
