LawnGuru credits never expire! If they're on your account and haven't been applied toward a service, they will remain there until used.
However, the maximum amount that can be applied towards a single service is $15.00. The amount in which the credit was given will be the amount taken off your service.
i.e: if a customer has two $10 credits, they have a total credit amount of $20. In this case the customer will receive $10 off of two services. Credits can be stacked but not combined.