I'm an experienced DIYer now, but when I first started vaping, I ordered a bunch of flavors mixed that way. They're still around here somewhere! Hopefully your results will please you more!
The first steps in DIY are choosing your flavors and ratios, so what you're doing is like "DIY Lite" in my opinion.
The thing is, if you ask for 2 parts strawberry + 1 part menthol, they will assemble that by volume (or maybe weight, but little difference there). It's up to YOU to know that menthol is a very strong flavor, and strawberry tends to be far weaker. That 2:1 is going to be really strongly menthol! Just HOW strong depends on what brand of flavoring they're using, and they probably don't tell you that. So how to get a decent bottle of juice from this process?
Two ways: trial and error, or follow in the footsteps of others. You can order 2:1 this time, 4:1 next time, until it's the way you want it. Or there might be user-posted recipes there that others have enjoyed. It's no guarantee you'll like the same thing, but it's something to start with.
DIYers follow the same process and might start with 2:1, taste, and adjust repeatedly until it's right. Then shop around for a different strawberry flavor, lol! We all end up relying on the mail service, one way or the other.
My advice is to keep handy some premixed liquids you enjoy, and don't get your hopes too high for great results the first time on this service. Order a few and keep track of the numbers you used to adjust your order next time.