Apparently the "A" necklace was a gift from a brand that I've never heard of.
Gifts have always been a bit of a grey area - whilst the RF trumpet that they pay cost price for everything, that's never been true. Laudner has always gifted their bags to the Queen for example, and continue to gift her any new colours or styles every year.
And like I've said on other threads, I believe a lot of Kates Kiki earrings are gifts from the brand. And Meghan's Monica Vinader is 100% gifted - they are a brand that send a lot of product out to Instagrammers so gifting is part of their PR.
They are already walking advertisements but I can see this already loose protocol becoming so relaxed that the Royals become moniterised by brands.
After all, Kate is already a walking advertisement for Jenny Packham and McQueen and Meghan is an advertisement for Givenchy. I imagine they are charged very little. The brands would be mad. They would whittle that bill down to the nth degree. It may be that they only pay a nominal cost towards those custom gowns. It's in the brands best interest.