In Shakespeare’s Love’s Labor’s Lost he writes:
O, thy letter, thy letter! he’s a good friend of mine: Stand aside, good bearer. Boyet, you can carve; Break up this capon.
It was used like poulet in French for a love letter. “Break up this capon” i.e. “Open this letter.”
Capon from the OED
†4. A billet-doux. Cf. French poulet ‘a chicken; also, a loue-letter, or loue-message’ (Cotgrave). Obs.:
Poulet as a love letter has an interesting etymology from OED:
Etymology: < French poulet chicken (13th cent. in Old French), written message (1556 in Middle French as poullaict ), love letter (1588 in Middle French): see pullet n. The origin of the French transferred uses is unclear; they perhaps arose from the comparison of the shape of a folded letter to a chicken wing.
†1. A love letter; a (neatly folded) note. Obs.
1691 W. Sherlock Dialogue between Dr. Sherlock, King of France, Great Turk, & Dr. Oates 2/2 Will you send it in a Basket as a Token of your pure Love to absolute Soveraignty, or in a Billet Dieu [sic], or in a Poulet as I us’d to do to the Nuns at Salamanca?
1751 Ld. Chesterfield Let. 28 Jan. in Lett. to Son (1774) II. 93 If you were to send a poulet to a fine woman.
1847 Thackeray Vanity Fair (1848) xxiv. 206 He..sate down to pen a poulet..to Mademoiselle Aménaide.
1894 S. J. Weyman Man in Black ix Even the Commissioners..found their doors beset at dawn with delicate ‘poulets’, or urgent, importunate applications.