>a. Even though they pay only 20 hours of my time, they assume 100% availability. b. It gets awkward with them when they suddenly want to increase the hours but

>a. Even though they pay only 20 hours of my time, they assume 100% availability. b. It gets awkward with them when they suddenly want to increase the hours but I have already committed my remaining hours elsewhere. You need what's called a retainer agreement. It will specify an exact amount of time they have to pay you for every X, as well as specify availability and charge for it. It will literally specify what availability you have to give them, and how much they are paying, specifically for that availability, and that you are working for other clients and will not necessarily be able to change the terms of the agreement without several weeks/months notice. (Additionally, retainers are often paid in advance, not arrears :oD, and charged wether they have work for you to do or not)