You said it was a debit card? You probably just need to call your bank, talk to their internet/fraud department, and have that seller authorized, or added to the white list. It's pretty common with seed shops, especially overseas the bank computer will block stuff sometimes, and sometimes randomly, if they think it might be fraudulent. But one quick call always gets it sorted out and authorized.
I been using a reloadable greendot..the one that they mail you one with your name on it after you register it online. You can keep that card like a regular debit..expires in a certain given time. The card you buy at the store doesnt work for international orders..the one they send you in the mail as your permanent card. That one does.