(1) Where an offer invites an offeree to choose between acceptance by promise and acceptance by performance, the tender or beginning of the invited performance or a tender of a beginning of it is an acceptance by performance.

(2) Such an acceptance operates as a promise to render complete performance.

Comments:

b. Part performance or tender. Where acceptance by performance is invited and no promise is invited, the beginning of performance or the tender of part performance creates an option contract and renders the offer irrevocable. See §§37, 45. Under Subsection (1) of this Section the offer is similarly rendered irrevocable where it invites the offeree to choose between acceptance by promiseand acceptance by performance....But unless an option contract is contemplated, the offeree is expected to be bound as well as the offeror....

d. Preparations for performance. As under §45, what is begun or tendered must be part of the actual performance invited, rather than preparation for performance, in order to make the rule of this Section applicable. See Comment f to §45. But preparations to perform may bring the case within §87(2) on justifiable reliance.