PageSender Fax Center 4.2

With PageSender, fax or email anything you can print. Use the built-in PageSender Fax Center to manage and automate your fax activities.

Set your printer to PageSender, enter the fax number, click Print and it's off.
Drag your customer group into the address list, press the Print button and it faxes them all.
PageSender is fully AppleScriptable and includes just such an example that is customizable to your needs.