I use Downpour. They all kind of have the same pricing service. $12ish for a monthly one credit, buy more at the same price. Downpour lets you either use their app for syncing or just download the MP3 and/or M4B (a format similar to MP3 but with chapter stops for books) to use however you'd like.

Though I'm not sure it supports gifting. Someone else suggested Libro.FM which is very similar but I know does have gifting.

I avoid Audible personally, they've historically taken a huge cut from authors. I can get basically the same deal everywhere else. If you're curious check out Brandon Sanderson's various posts or media releases about the topic.