What does being a professional have to do with giving the customer the courtesy of knowing about a price increase?
If there is NO contract for services provided AND the price increase is something in line with cost of living percentages I simply don't feel it is necesary and I don't do it. I would notify them IF the increase was beyond a cost of living approximate percentage OR the job parameters have changes for any reason causing the price to escalate. I wouldn't want an ugly surprise.
When the gas company raises their rates, ever notice that there is an extra piece of paper detailing their new rates in the bill a few months before the new rate takes affect?
A gas company is a regulated public utility. All regulated industries have different rules than the rest of us.
Maybe the problem here is in the perception of a price hike and how often it is done. If you cut a lawn for $30 for 5 years then bump it to $37.50 which is a 25% increase, you may want to notify so there is not sticker shock. If you bump the cost $1.50 every year, I personally wouldn't sweat it.
For every example someone cites of posted prices or notification there are an equal number of examples where increases just are made. Even when prices are posted for us they are the standard prices, not an increase. You wouldn't be aware of an increase unless you bought the item regularly or it was an item of importance where the price is filed to memory. When we do see posted price increase on something we need, do we not buy? Most of the time we do. The increase most likely would only deter a discretionary purchase that could be delayed.