a big part of shop work is the area you live in,bristol city center must have over a 1000 shops whitin a 5miles radius,add that lot up plus the windows of offices above the shops and you have a nice days work,3days later your cleaning the same shops.
i do a friday/thursday morning doing shops,yes your right not a great pay i could earn the same doing houses,but i like the different workand i dont start before 7am doing them and cause its a friday i finsh at 2/3pm,but these shops get done no matter what the weather and all pay on the day.