Let me explain this carefully, since the title can lead to some misunderstood:

You want to buy an item from Secret Shop. Let's say it is a Void Stone. You have the Void Stone in your Quick Buy, you also have the money to buy it, but you are away from the shop, so you send the courier. When the courier arrives at the secret shop, you need to SELECT the courier to buy the item. My suggestion is simply to remove the need to select the courier to buy the item, allowing you to buy the item with your hero selected.

TL;DR: If you are far from the Secret Shop (or side shop) and click to buy an item, and you have a courier next to the shop, the item is bought and goes to the courier, instead of not buying the item at all.