Giving your staff these would mean you're serious and effective on infection control and safeguarding: External Link/Members Only
Seriously good piece of kit in the East Asian preferred style for healthcare workers. Give your staff those and Matt Hancock would have you arrested.

It is overkill though and shop staff just wouldn't be happy wearing them. Hospital staff aren't comfortable wearing them.
Each retail setting has to have an individual policy tailored to suit, but based on good general principles, which will include how enclosed the premises are. This can be complex but may be as simple as having a window open in very confined settings.
You are absolutely right though that the wearing of masks, by shoppers in particular, has a significant and cost effective role to play, but despite this having being advised in shops, the uptake appears to have been small? For it to work best
ALL shoppers have to comply.
Not wearing one maybe has almost a 'macho' element to it? ... but also when a government says "we think its a good idea to wear a mask/facecovering but actually we don't think they work" then the lack of uptake is hardly surprising.