I have to start by saying I know very little about this particular series of stories. But I did try to see what I could find that fit within a "shirt/poster/keychain" kind of budget. Sadly, since I know about as much as you do about 7 Deadly Sins, I had trouble with even knowing what to look for. :/

I tried digging through Amazon to find something related to the Daughter of Evil story, the only work my Mothy I'm even a little bit familiar with. Somehow all I was able to find was this little guy. I have no idea why Amazon always has so many cheap mousepads. But dang, that art is cool.

I also managed to find this set of seven buttons on etsy with characters from the series. The pins use the seller's own drawings.

Beyond that, I have just one bit of advice. If he's into Vocaloid in general, see if you can figure out what his favorite "singers" are. (Since you mentioned something about twins I'm guessing he probably likes Rin and Len. They're enormously popular.) Once you know that, anything with that character's default outfit is guaranteed to be a hit. My own boyfriend is a huge fan of wall scrolls, and those can be found really cheap on Amazon and eBay. And all weeaboo trash love figures. They're pretty much plastic crack.