When you use "encargan" (although in this case it would be "encarga" because "producto" is a singular word and so should be the verb) you are using the present tense, therefore you're talking about an action that is happening. If you say "encargan" you are saying that you are ordering that product. It's a fact. When you use "encarguen" (again: encargue), though, which is a subjunctive, you are not assuming that is happening: you're basically opening the possibility. It's not something that is SURELY going to happen, but if any product is ordered in that month, there will be that discount. If. Basically the difference is that: "encarga" is a fact and "encargue" is a possibility. I hope this helps but this is my first time answering any question, so if I didn't explain myself too well or you just didn't get it, you can ask again ^^