What document should you provide to the vendors if you're only interested for a set fee for their materials?

An IFB is the appropriate procurement document when specifications are fully defined and the selection criterion is price alone. Vendors submit a fixed price bid, making it ideal when a set fee for materials is required without negotiation on scope or approach.

An RFP (Request for Proposal) is used when the buyer wants vendors to propose both a solution and a price, implying scope or approach is not fully defined.

An RFI (Request for Information) is a preliminary document used to gather market information from vendors before issuing a formal solicitation, not to obtain pricing.

A SOW (Statement of Work) describes the work to be performed and is an attachment to a contract or solicitation, not the solicitation document itself used to request a fixed price.