This is mostly due to preference in terms of appearance and value of the value add. As mentioned, for myself, i mainly focused on the ring and value adds related directly to the ring like designing and customization rather then vouchers and soft toys. Maybe cos i'm an engineer to very... practical ? Might not always be good cos some times girls also like not so practical stuff like soft toys etc so much know your girl and balance ba. Hahaha. Or buy the ring and buy the soft toy yourself seperately. After all, while it is cute, how many other girls go the same love and co soft toy already. hahahaha. Get some unique soft toy related to something that she likes instead is also more personal.
Marketing ba, everyone will tell you theirs is the best. Safest way is to learn how to view it yourself and evaluate. Failing that, JP or other private jewelers willing to provide you with scope images is the safest option as can post here for other bro's to evaluate
That said if going for branded cuts, as bro Draculav, previously posted, some branded cuts are very well cut like HoF and Destinee etc. All these round brilliant types are easier to evaluate and compare. Those special cuts or modified cuts like rose cuts, celestial all those, even with scope, unless something is very wrong we probably won't know what to look out for even if we saw it. And have to trust them and the brand. When buying a brand sometimes you don't just pay for the diamond, but also for the unique designs, such as the Lee Hwa Forevermark series which really does have some pretty interesting designs.
For this i would say, unless you know how to look at the diamond yourself, some level of trust is required. The cert is just a piece of paper, it can be faked. To check if the report is real, you can check the cert number online at the respective sites (GIA, AGS etc). But its just a piece of paper and could possibly be duplicated, so can check the laser inscription, and verify the inscribed number online. But once again, laser inscription is cheap... If they really want to cheat you, they can inscribe the number on another diamond of similar size. Overseas one possible scam is to give you the real cert but swap the diamond instead, then sell the real separately diamond as well.
As the saying goes, buy the diamond, not the cert. Only way to 100% know if your diamond is your diamond is to know how to see your own inclusions. BTW, if at private jewelers like JP you can request for them to try to find and show you the inclusions on your diamond if you want

But to do it yourself you will still need a loupe and knowledge on how to use and what you are looking out for.