Oh you can buy more inventory slots, but you will quickly find the cost spiraling out of control.
From what I've seen, you cap at 125.
Costs 25 token (real money currency) to get to 130.
30 more to get to 135.
35 to get to 140.
40 to get to 145.
45 to get to 150.
50 to get to 155.
So by the time you bought your 6th +5 slots upgrade, the price per 5 slots has already doubled. That's out of control costs.
The formula is basically for each box you buy, the price you pay is the price of the previous box +5 +the initial rate of 25.
Inventory upgrade number 4 costs 25+(5x3)= 40.
So inventory upgrade number 6 costs 25+(5x5) = 50.
Upgrade number 101 costs 25+(5x100)= 525 tokens. For 5 inventory slots.
I'm pretty sure my math is right, but feel free to correct me if I'm wrong.
Still want to buy your way to infinite inventory size?
After your 101st inventory size increase, you still only have 125+505 total slots. Some players have many thousands of items. And to reach #101 upgrade you've spent... I believe its 25,000 tokens to reach this point?