You want to make a positive impression on your supplier, so cast your company in the best light possible. Give the supplier an idea of the magnitude of your request, i.e., how much credit you're going to need.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Abc Company is <background>. <Additional details.>
<Reason and amount.>
Accordingly, please send us information on your firm's terms of credit and billing practices. Thank you.
encl: <List of enclosed items goes here>