The grocery store, gas station and coffee shop do not send an invoice after work has been performed without prices already being posted. Each time you make a purchase, it is a choice to buy.
I believe you should notify your customers before a price increase - so it's their choice as to whether they want to buy. We send out our renewal/update letters (our contracts are continously renewing) each February, letting customers know that effective March 1, prices will be xx.
Lawn Lad, Inc.