How do I get apps to not crop my photos when trying to print them

I’m trying to print 4x6 photos and don’t want my photos to be cropped.

Previously I’ve been able to add a white border to the photos that aren’t that ratio when buying the prints from the store but when I try to buy them online I don’t get that option.

I’ve tried Shutterfly, Walgreens, and CVS.

I’m starting to get frustrated and am not sure what to do, I’m worried that if I go in person it’ll take a really long time and I won’t get the discounts I get on these apps!

Any advice would be appreciated!! Thanks!