in the donation section if u donate $50 you get 300 (3000 Maximum) inventory space.
What does this mean?

You can get 3000 (for Steam Items only, for other game items you need to provide BP expander) for around $~65 in total donation.

"3. This is the maximum limit of your shop inventory items, including items from all Steam games, Steam community items and Steam gifts. VIP can apply Backpack Expander to increase this limit. Every donation of $0.6 beyond $50 also increases the limit of Steam community items and Steam gifts by 100."