A company I know recently asked to me to help them prepare a proposal. As we talked, they realized that while they had talked to the customer and the customer was interested, they hadn't sufficiently defined the scope in order to write a proposal. They knew that they could meet the customer's needs, but they couldn't figure out how to price it in a proposal because there were too many alternatives and too many unanswered questions. Their sales process succeeded in getting the customer's attention and interest, but broke down when it came to preparing the customer to receive a proposal. Without a defined scope of work, they couldn't write a proposal because there were too many possible ways to structure a deal.
Even if they had a defined scope of work, they hadn't discussed with the customer what process the customer would follow to receive a proposal, evaluate it, and make an award. Even though it can be difficult to initiate, it is important to have a conversation with your customer about what they would like to see in the proposal --- what the scope of services should be, how it should be priced, and how it will be evaluated. This conversation will help prepare them to receive your proposal and provide a common basis of understanding for how the process should work.
A conversation about how the customer would like to move forward can be valuable in other ways. You may find that a proposal isn't even necessary, that the customer has accepted your qualifications and is prepared to sign a contract. Or you may find that the customer is just browsing and is perfectly willing to accept all the information you care to provide, but has no real intention of closing a deal. This conversation gives you a chance to evaluate how serious the customer is. It can also give you a chance to influence the proposal in ways that can work to your advantage and make things more difficult for any potential competitors.
Even if the customer is serious, if they have to write an RFP it can be a serious impediment to closing a sale. If the customer has never prepared an RFP and has no process to evaluate a proposal, this conversation may be the only way they will get sufficiently organized to prepare one. When sales drag on and never close it is often because this preparation was not done. The customer doesn't know how to reach a decision and close a deal from their end. When a customer receives a proposal that they don't know how to evaluate and have no criteria to evaluate it against, they often fail to act on the proposal. If you want the sale to close, you need to make sure that the customer has this process in place.