Allow customers to decide if they want Image on Front or Back of shirt. Just add another dropdown after size and color.
I also sell on redbubble. This is an option on heir website and honestly, many times, I wonder if I placed the image on the back instead of the front, if that would result in more sales, and vice versa
I'm an 8K level seller, so I can easily add another listing to the opposite side but don't really have the time to do that . Nor do I want to waste too many slots not knowing which side will be more successful. For people at a lower tier, this would be a fantastic way to increase sales with less effort
For customers, it would give them more options to tailor the design to their liking which equates to more sales.
For merch, you are reaping additional royalties as well and as the template is the same size, regardless of design placement, there shouldn't be too many workarounds needed to accomplish this as far as setting up the option on the listing.