That definitely does not look like a hydrangea bloom. If you were expecting to get a Hydrangea paniculata (sometimes called a hydrangea tree), they shipped you something else.

I can't be of much help to what it is either but would like to know where you got it. I'd like to get one.

Thanks.

I posted on the other thread you showed this on that I believe it is Button Bush (Cephalanthus occidentalis)

Agree with hcmcdole....definitely a button bush.....loves super wet areas......more of a swamp type plant.....
You may consider keeping it....they are very showy, especially if you prune it to keep it bushy.....