See it from programming POV:
The bag is closed during a game - no change of clubs. Here, the balls are part of the bag - no change of balls, too. As a courtesy to you and as a benefit to the company, you may buy "more of the same" but not add anything different.
If you need an analogy, see the inventory as your garage - unavailable, and the cart girl only has your ball type available when she comes to you.