Firstly, I'd contact the seller. They may just have missed a bit, be embarrassed and stick it in the post as quick as possible! 😊
On the other hand, if you get no joy from the seller, you have 30 days from the sale to open a return, under the subject 'item not as described'.
The seller will have to send you a 'returns label' (so don't worry about it being a heavy item to send back)
Don't let the time go over that 30 days..... and don't let the seller 'charm' you into waiting over that time, no matter what they say😎