JLPT1 G7 S15 F1412
舎 here was originally 金 metal, which when added to 甫# (spread) yielded the meaning of a metal bracket fitted precisely (= spreading flat) into a gate. (The present-day character 鋪, however, means "lay out; spread; pave.") For 舗, the idea eventually became that of goods *spread flat* for display/sale (compare 販) in a simple structure → *sales* → *store*; *shop*. Note a variant form, 舖.
[Full Kanji Etymology List]