Can you please check my '?

[Shouldreturntoclass]

'for everyone's benefit' is the apostrophe correct and..

'thank you for being a Flowers customer' Flowers being the name of the company. Does Flowers need to be Flowers' really appreciate your help, sorry for being a dunce.

First one: it means 'for the benefit of everyone' - so that 'of everyone' tells you it's a possessive, and you put apostrophe + s on the end. In short, you are correct.

Second one: I think there's a bit more going on here.

It's true you could have something like "we'd love some of Tesco's customers to shop here", so you would say "we'd love some of Flowers' customers..."

But on the other hand it seems fine to say "thank you for being a Tesco customer", so Tesco becomes a noun attaching to another noun rather than a possessive. So, I think "thank you for being a Flowers customer" without the apostrophe is acceptable.