If you're promoting your stuff on the internet, you need to write it in a way that brings potential customers to you. People have so much information thrown at them, your words are vitally important.
It's not enough to just write out the facts without making them sound good. Get them right and you could get a sale. Get them wrong and you're just wasting both yours and the other person's time.
What about if you had a laundry service?
You pick up the dirty stuff, drop it off and only charge a fixed price - £20 per bag, where each bag holds 15kg of stuff.
Think about why someone would want this service ... from my point of view, the big benefit is the pick up and drop off. So I don't need to waste my time sat in traffic for the sake of some dirty washing. But to someone else, it might be the fact that it's a fixed price - you always know what you're going to be paying. And to a third person, it might be that the bags are big, so you can get the whole lot into a single load.
If you were running ads (or putting together a flyer or doing a sign), you'd probably want to run at least three - one for each of those benefits. As each one will attract different people.
So pick one and think - why would someone want this? "No time to do get to the laundrette?"
Then think - what would the ideal situation be? "Wouldn't it be great if you didn't need to waste your time sitting in traffic, just to get a load of dirty washing clean?"
And then, and only then, do you present your solution. "We'll pick up your washing, get it all done perfectly and drop it off for you. So you can concentrate on more interesting things"