Your company may have a generic letter that you can use, or you may need to create your own. If you need one, here are some ideas to get you started. Some people write this out in paragraphs, others prefer to do it in point form. When you create yours, you will figure out what method works best for you.
Mine start off with how excited I am to be doing a party for her on X date at Y time. This is important as people have a tendency to forget.
From there, you may want to go into information about who to invite, the hostess benefits (generic) and if they need to provide you with a guest list.
Next, you will want to give them information in regards to tax and shipping rates and other information about placing orders. You will want to include what type of payment information that you can accept as well as to whom to make cheques out to.
You could offer information about methods that they can use to get some orders beyond their party, and you may want to offer incentives for orders colleted at this point.
If you do on time draws, you will want to mention this in the letter at some point, as well as anything else special that you may do. This would also be a good point to mention keeping the food and drinks simple.
Finish off the letter with a note as to how much in advance you will be arriving and what you require for setting up.