When printing double sided products, like business cards, we do what it's called head-to-head where the top of both sides is alligned.
If you send us your files this way, the backside is going to be printed upside down.
This way both sides will be printed upright.
When you send us one side vertical, we rotate it 90° to the left, so make sure it's facing the right way otherwise, it will print upside down.
If you design your product vertically, you can send it to us that way and we'll make sure to flip it correctly before printing.
However, if you choose to flip it by yourself before sending it, please make sure to flip them as the next image shows to assure both faces are printed in the right way.