I was soon invited to submit an introduction letter in order to be assessed as a potential supplier and I have to admit that I've never used one before. I would appreciate a hand in this topic and I promise to let you know the outcome! Thank you;)

Well, It's pretty simple.
On a letterhead,
1) Start of with an introduction of your organization (name, expertise, years in industry... )
2) What is your forte, your USPs!
3) End with a small note stating, why you think you working along with them will be good for both of you.

Well, to be honest, there's really a lot that you can do with an intro letter. I think what I've mentioned can be used as a rough guideline.

Hope this helps!