Currently, folders cannot be merged (we hope to offer this feature in the future, though!).
However, you can quickly move all of the contents from one folder to the other and then delete the empty folder.
To do this, use these steps:
Tap into the folder you plan to delete
Press and hold an icon on one of the receipts to select it
Once selected, tap the double checkmark at the bottom of the page to select all
Tap Edit
Deselect the current folder and select the new folder where you want to move them
Tap Save
To delete the empty folder, navigate to the space where the folder is located and tap the pencil icon to enter edit mode. Once in edit mode, you can tap the folder and then tap delete.