Opodo and eDreams are notoriously bad at processing refunds and for making spurious subscription charges. I use Opodo, but you need to be aware of the potential issues. They also use some very sneaky browser recognition techniques (not cookies) and gradually push the prices you get offered up, so you start with very low prices and gradually you get closer and closer to direct fares. Swapping browser gets you back the low fares, or use something like Tor browser which usually defeats the technology Opodo use.

Also you have to be very careful not to inadvertently sign up for Opodo Prime - lots of reports of charges being made after subscriptions were cancelled, though I've never had an issue. This was the agent I found with negative prices at one point (fixed discount with no floor level), and some very inexpensive fares in 2020/1 (£0.99 edi to malta) but not as good as it was.

It's worth doing a scan of twitter to look for complaints about OTAs, you'll never get anyone who's spotless but you can get a feel for reputation.

When Opodo works, it's fine, they ticket ok. They do eventually refund on cancellations though you may be picking at them for years or encounter admin fees. Approach with caution.