itch.io

You're welcome :)
For the item system, I can think of two options:

• Have the enemies drop credits only. You could put everything in the shop from the get go and let the player anticipate, plan and hopefully salivate on the juicy upgrades , and motivate them to collect all the credits. Have the credits expire after a few seconds (like in Surwave) if you want to add some extra pressure & tension.
• No shop. You salvage the items that the enemies drop randomly. You'd probably want to tie the quality (e.g. weapon strength) and the quantity (e.g. ammo) to the tier of the enemy.

The first one gives the player more direct control and choice on how to spend their money and the second one adds an immediate reward and progression mechanism and randomness.