In the Payment Options card on the right of the proposal, switch on Disable Tax and/or Disable Delivery Fees and save. Tax and/or delivery are then excluded from that proposal's totals and checkouts. Both toggles are logged in the Activity tab as Disable tax changed / Disable delivery fees changed.

Note: these two toggles are proposal-level and take priority over anything else — in the current code, when a proposal loads into the cart it always explicitly sets the cart's tax/delivery-fee flags to match the proposal's own Disable Tax / Disable Delivery Fees checkboxes, which runs after the customer's own account is loaded. So if a customer's account is separately marked tax-exempt but this proposal's Disable Tax box is left unchecked, the proposal will still charge them tax — the account-level exemption does not carry over automatically. Use Disable Tax on the proposal itself for a tax-exempt customer's proposal to be tax-free.