In the first part of the letter occupying no more than one paragraph, write that the company you represent, and welcome on behalf of the leadership of the alleged partner. It is appropriate to use phrases such as "we wish prosperity to Your business, from the name of a certain company expresses its respect". Be sure to include the name of your organization. Remember that spamming in any firm comes more than a dozen letters with the offer to conclude the contract, therefore, the addressee must understand, whose offer it is.
In the next part of the letter will remind potential partner about what your companies already existed prior arrangement on a particular contract or service. You can use such momentum as "In continuation of our talks...", "We have considered Your wishes regarding...". Make it clear to the addressee that the offer of cooperation is not formulaic and basic for all clients, and designed specifically for this company, and takes into account all peculiarities of this business.
In the largest part of the proposal indicate what services or goods you offer, what they cost, delivery time. Easier to read information presented in tables. Offer the client several options with different price, quantity, and scope of services that it can select the most suitable combination. We would highlight the benefit for the customer, signing the contract with your organization. Here you can specify discounts, special offers for regular partners, the possibility of concluding contracts for the provision of additional services or the supply of other goods for extra-favorable terms.
The last part will be devoted to the wishes and parting words. Use standard phrases such as, "Hope for mutually beneficial cooperation...", "best wishes...". Be sure to subscribe and leave your contact information, so that in case of a positive decision the client is not looking for a phone or email address by which you can be contacted.