The affix śnā comes after the roots of the kryādigaṇaḥ (9th Conjugation) when a sārvadhātuka affix follows denoting the agent.,
This debars शप्. As क्री + श्ना + तिप् = क्रीणाति 'he buys'. So also प्रीणाति 'he pleases'.,
क्र्य्-आदिभ्यः श्ना
Adhyāya 3 · Pāda 1 · Rule 81