Have 3 or more of one item, (Berry) and 1 of another item. (Potato).

When shipping to the Union, select the berry, press down to set the amount being shipped to 3. Now press B. Select the potato, and press down. The amount being shipped should be at 2. Sell the 2 potatoes, and you will now have 55. Select the again, and you can hold down until you get to 99.

This will work with any two items, but you have to select amounts by pressing down.