Cool mechanic! Wow does the difficulty ever ramp up quickly! I found in level 3 I could skip using the boxes entirely and just take a break from carrying for a moment after making the first drop. Didn't get far into level 4 before I felt like tearing my hair out! In a good way though! Might be cool to add (if you haven't already) some kind of in-air collectible that recharges your strength. Would make for some crazy levels.