To provide customers with options on your estimates, first have the options ready under Estimate Templates.
To set up Estimate Templates, go to: Settings > Customizations > Estimate Templates
Once the options have been selected go under Estimate Templates > Proposal Templates > + Add New Proposal Template
Follow the steps and drag the estimate options into the options present by the Proposal Template.
To send a proposal, Create Estimate > Select Proposal Template > Add Customer > Adjust and Send.