Lewis & Short

per-grātus, a, um, adj., very agreeable, very pleasant (class.): litterae, Cic. Q. Fr. 3, 1, 6, § 23: id mihi pergratum perque jucundum erit, id. ib. 3, 1, 4, § 12; id. Att. 10, 4, 1: pergratum mihi feceris, si, you would do me a great pleasure, a great favor, id. Lael. 4, 16.
In tmesi: per mihi, per, inquam, gratum feceris, si, etc., Cic. Att. 1, 20, 7.