I generally refuse to use any shop that doesn't accept cards but, although 'minimum spend' requirements used to be common, I can't recall encountering any at all within the past few years. So there must be some very odd shops in your area, TC44!
In the early days of debit cards banks, did tend to charge retailers highly when they accepted cards for small amounts, so it made sense for retailers to insist upon a minimum spend. These days though the charges made by banks are far more reasonable (
https://tinyurl.com/y2oeps4f ) but perhaps some retailers haven't yet realised that?