Shoebox would absolutely only use your cellular data if you allowed for cellular backups during the initial set up.
If you had initially allowed this but you have changed your mind about data usage you can turn this setting off and limit photo upload to using WiFi:
On Android, we highly recommend you stick with the default "Data Saver" setting. It waits for WiFi to sync full resolution photos, but it syncs small previews over your cellular connection so that your photos are immediately backed up in case you lose your phone. It uses a relatively small amount of data and it's quite fast.
Keywords: WiFi, Data, Cellular Data, Usage, Settings