Here are 3 ways to go about avoiding this from occurring.
Option 1 -
Set it to a seat. Set each item to a seat does the trick.
Option 2 -
Instead of the +/- just tap! Tap the item X amount of times you want to add the item.
Option 3 -
If you charge 1 item, then come back to the check and charge 1 item again this will also do the trick.