Dear Felix
We have been pleased to do business with you for the past [DURATION, ex. six months] and we look forward to dealing with you in the future.
I am confident that you are pleased with our history of prompt payment within the [CURRENT TERMS, ex. $3,000 maximum, 30-day] terms you have extended to us. To facilitate the volume of business we foresee doing with you in the future, however, we request that you [REQUESTED TERMS, ex. extend our credit limit to $7,000].
Please contact me at XXX-XX-XX if you require any additional information to process our request. I look forward to hearing from you.
Regards
Lydia