as far as I know that 사다주세요 is from 사다가 주세요 ( ~다가 주세요 )which means to do something in a place and wait the result in another . and here 사다주세요 means : buy something for me and bring it to me . while 사주세요 means only ' buy that thing for me " hope you find this helpful.. good luck!

I'm going to say...a typo.

I see 갖다 주세요 used a lot.