Linnworks

Massive time saver

A very comprehensive and flexible system. Some ways of doing things can be a bit quirky and catch you out but it is really a question of making sure you know what you are doing and exactly how they expect you to do it. Some familiarity with scripting/pseudo coding is useful (or just sub it out to them) to get the best out of the automation. There's a lot of help via the forum and videos if you don't go for training. Apparently, you can get custom training for £40ph and this can fairly much be a "show me how to do these bits that I'm stuck on".

As far as I can see, Linnworks is the de facto standard thus offers the most comprehensive integration options as far as I can find.