Reference article
Maximum Level Upgrade
A shop upgrade that raises the shared skill and energy cap by ten levels.
Buying the upgrade
Visit the 
Duck Shop with at least 50 
Coins. Each purchase raises the maximum level by 10. Thirteen purchases raise the cap from 20 to 150, costing 650 coins in total.
What changes
The upgrade changes the ceiling for running, flying, swimming, climbing and energy. It does not add XP or instantly raise the duck’s trained levels.
Original Flash
Every maximum-level upgrade costs 50 coins in original Flash.