Build your sample work once, reuse it across as many proposals as you like.
Go to Studio → Galleries and create a gallery — give it a name (e.g. "Candid Wedding Highlights"), then upload the photos you want in it. Organize separate galleries by style, event type, or whatever grouping makes sense for how you pitch clients.
While creating or editing a proposal, pick a gallery to attach — the client sees it alongside the quote, giving them sample work to match against the package you're proposing. You can also show a portfolio button on the PDF linking out to a fuller external portfolio, if you have one.
A gallery isn't tied to one proposal — attach the same one to as many proposals as you like. Update the photos in one place and every proposal that references it shows the update.
How many galleries you can create and how many photos each can hold depends on your plan — check Pricing for the exact limits on your plan.