‘Discount’ is a bit better than ‘off’ because it’s more formal, and that’s more appropriate when describing rules.

The correct preposition is 'towards' but nobody would be confused by 'to'.

The correct sentence is: "This order counts towards your 10% discount." In this context, you want to convey that the current order contributes to the accumulation of purchases that will eventually result in a 10% discount. "Counts towards" is the appropriate phrase to use to express this idea.