Do nothing
Use 1 xp to fix 1 durability (instead of the normal ratio of 1 xp/2 durability
Fix the item for free (this is what mc does)
Use 1 xp but add a tag to the tank so the next time it happens it is done for free so the ratio is kept (please no)
Use a randomiser to choose between costing 1 or 0 xp so that on average the ratio is kept (please also no)
1
11
8
1
1
1