Dear :

We plan to increase our purchase from your company and place a number of substantial orders for [goods]. Since we have already dealt with you for quite a long time, we would like to know if we are eligible for a $ credit line.

Over the past months we have ordered fairly regularly from you and settled promptly. We believe this has established us as a reliable customer, though we are willing to supply references if you desire.

Hope to hear from you soon.

Yours sincerely,

[name]

[title]