If your voucher covers the shipping cost, per Groupon, it will absolutely cover the value for Economy shipping! If you are on the page requesting to select a shipping option...
...this is only showing you the VALUE of each shipping option (similar to how in your cart, you are still shown the value of the print, even though your Groupon covers it).
If you choose a quicker shipping option, you will need to pay the difference.
When you proceed to the next page of checkout, after selecting your shipping option, you will see that the total does indeed cover the full cost of the print and Economy shipping (leaving only taxes and additional services to be paid).