Ask them to use your seat. If it was bought online and you've never heard of the brand, it might well not conform to EU standards. There are a lot of counterfeit products on amazon, someone was killed in the US when they were in a cycle accident wearing a helmet they had bought on amazon and it claimed to conform to the US safety standard but actually it was a cheap import and did not. Nobody is checking every single product that is ordered directly from overseas.
It's a bit of a misconception that the cheaper/generic seats will "snap" if they are in a crash higher than 30mph, although some of them are very flimsy, but I would be really wary of some random amazon Chinese no-brand. Think about the phone chargers that cause fires, children's toys that just fall apart, the fake Ergo carriers etc.
This one is pretty common/popular at the moment, sold under various names (reebaby, reecle, bonio, mycarsit among others) - you can check if it's the same model by looking at whether the approval number on the orange label is the same (049564)
www.which.co.uk/reviews/child-car-seats/lettas-murphy-916
It does seem to be legitimately approved but the approval was done in Czech Republic which might be more lax? Or it meets approval but is just of a lower standard compared with other brands.
You also tend to find that the instructions on this type of seat are confusing - there was a thread here just the other day asking about the Puggle Lockton and many people thought you had to install with isofix, top tether and seatbelt (not true) and found that you can't spin it when it's properly installed (true) - I don't know for sure obviously but I believe these seats are made for the Chinese market where many people don't have top tether and although they make them with the tether so they pass the testing, they don't actually expect people to use them. You should, 100% use a tether if the seat comes with one. The whole thing could just break apart or hit the seat in front if you do not.
It's a bit of a disaster for safety as an improperly used seat could sometimes be less safe than no seat at all.