My bot's tf2 inventory isn't full, it has 300 empty slots but shop inventory shows almost full because of steam gifts. So i must apply backpack expanders. But still have free slots, why do we are forced to use backpack expanders?

Quote from "Premium" page:

"VIP can apply Backpack Expander to increase this limit (up to a maximum of 2000). Every payment of $0.6 beyond $50 also increases the limit of Steam community items and Steam gifts by 100. Note that the effect of BP expander and extra payment does not stack. The one that precedes the limit is taken into account."

So you can apply what suits your need.