There are two important advantages to landing a flat rate client. First, he or she obviously has faith in your ability to produce decent content, so you can feel good about your work. Second, if you're struggling financially, it's great to be able to count on a certain sum each week or month, even if you're still bidding on other jobs to make ends meet.
There are disadvantages to this type of arrangement as well. There are clients who, once they have you in a flat rate relationship, believe that they own your every working hour. You have to watch out for clients who try to squeeze as much work as possible out of you, because you could end up writing at a per-word rate that's less than what you charge to ordinary clients. Also watch out for flat-rate clients who are unreasonable about revisions and rewrites. One revision is fine. Multiple revisions, or one or more complete rewrites on top of your regular workload for the client is not.
Here is what I would recommend if you or a client initiates a flat rate relationship. Agree up front on a maximum word count per pay period, and agree on a per-word rate for work you do that exceeds this. Your flat-rate client doesn't own you and doesn't own all your time. Try it out for a month and see how it goes.
If it looks like something you can live with, have an actual contract written up. Contracts can be signed electronically online. Be aware that if your client is located in another country, you could have trouble getting him or her to live up to the terms of the contract should things go sour. But the contract itself lets the client know that you take your work seriously and he / she should too.
Don't be afraid to stand up for yourself if a flat rate client tries to take advantage of you. If you can't work things out, end the relationship. The fact that you were offered a flat rate arrangement is a pretty good tip-off that you do good enough work you won't have much trouble finding other clients should things not work out.
Also beware of putting all your eggs in the flat-rate client's basket. Having even one other client on the side can make things easier if you have to end the flat rate arrangement. Flat rate arrangements for content can work out well. Just make sure to protect your interests as a professional writer before you agree to this type of relationship.
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