Do you have a smartphone? SSI makes it easy. They don't charge extra for a digital card.
Download the MySSI app, and login with the same credentials you used for your online training. Instructions are for iOS, Android may be different. Click on the 3 bars in the upper right to access the menu. Then select "Programs & Cards." Make sure the Cards tab is selected, and you should see your open water cert. This should be accepted as proof of certification.
Surprising, though, as the SSI shop I use prints the cards right in the shop. When my daughter got certified, the printer was down, so we had to wait a bit for the physical card, but the online one was available immediately.