File pulldown menu -> Import -> battery file
File pulldown menu -> Export -> battery file allows you to send it back.
This works for most GBA flash carts. Certain carts when used with things like AKAIO will have an extra header added onto the save to allow AKAIO to work more easily, you will need to remove this.
Alternatively you may also have to pad out or trim some GBA saves to match what VBA thinks it should be.