All startups will at some point have to write a proposal.
It could be a proposal for partnership, acquiring a warehouse, or a pitch for a large contract. In any of these situattions, you will be asked to submit a written proposal.
I will give you a very powerful method to write proposals by well established guys called Pugh and Bacon who have written the definitive book called Powerful Proposals.
So here is how you do it.
Two things to research thoroughly are:
Against each requirement of your customer, list all your and your competitors strengths and weaknesses.
You can do this by creating a table with rows listing your customers' requirements and the columns can be "Your Strengths", "Your Weaknesses", "Competitor Strenghts" and "Competitor Weaknesses".
Then using this table as your guide, start writing your proposal.
For each requirement of your customer, create a paragraph and write the following:
Highlight your strenghths. Don't be bashful. Show how your product features can help the customer and add some proof to make it convincing.
Example: We are experts in AI/ML software as you can see from our recent customer testimonials that show how they have reduced manual effort and human error rate by 50%
Mitigate your weakness. Don't hide it but acknowledge it but defend it by saying how other factors can make up for it. This is called mitigating your weakness.
Example: Other providers have more features but our software is more mature as can be seen from our 25k downloads and 4.5 star ratings.
Neutralize your competitors strenghts by making some points on how despite their strength, you can provide some compensatory features.
Example: Others might be able to do it cheaper but our software is highly secure as it is compliant with ISO security standards.
Ghost your competitor weakness by casting doubt on that particular weakness.
Example: Some vendors may claim an "end to end solution" but one should check if the components used are open source (shareware) which can compromise security.