You are only charged a single 50 lending fee per visit to the store, even if you purchase multiple items. For example, you may purchase 4 × Health Potion using your entire 200 allowance and after leaving the store, you will owe 250.
The conditions for leaving the store is the same as the conditions that disables your ability to undo purchases, and is tied to both distance from the shopkeeper as well as combat status. A few other effects and triggers will also classify you as leaving the store.
The maximum loan is 155 − ∞ (based on minutes) Gold.