this will help in eliminating the error msg
when you c/p your receiving btc addy into their site page
'backspace' (erase) the last character then TYPE it back in
example: c/p this --> 1BcGpZpZXTP4XikMKgyoBJCUN2o1LhmaRK
backspace (erase) the last "K"
then type it back in "K"
then submit