They utilize the blockchain.info APIs, however from what I could see, this seems to query the payment status once every x seconds from the client-side, which ... works ... but is not ideal, since there can go stuff wrong, for example if the client closes the browser, for whatever reason, but paid...
Cost-wise: If you have your own hosted solution, you're not really looking at more server-cost than you already have, however you'd have to pay once for the development and the integration. If you only have one domain and do not process more than 300k$/Month in BTC-Sales, you are probably way better of with Bitpay, which is 30$ / Month. Plus with Bitpay you have the option to get the payout in FIAT. (I'm in NO way affiliated with Bitpay, this is my honest opinion)
-> I cannot really give any advice without knowing any numbers(which you shouldn't share here
) but it mostly depends on your monthly BTC volume, if the 30$ account works for you, I'd say stick with BitPay for now.I do work like that and I could build such a solution for you, or integrate an available solution like the one mentioned above.
So if you are still interested or just look for further advice, feel free to contact me via PM.
