@YourBrightOpalSeal
I don't think they're saying that a volunteer's time and work is less valuable. It's just a fact that a charity shop's business model has always been built on the vast majority of the staff being volunteers. Having unpaid volunteers as the majority of the charity shop's workforce is the whole point of a charity shop; it's what the model is founded on.
And the volunteers understand this. I don't think any volunteer feels exploited simply because they are an unpaid volunteer. (I am a volunteer myself and do not feel exploited at all.) Volunteers know they will not be paid and freely choose to volunteer their time.
I think you also need to take into account that volunteering has a lot more flexibility than a paid role, which is again a point against exploitation. Volunteers don't have to be there if they don't want to. They can take time off whenever they like and for as long as they like. Some years ago, the shop I volunteered in had a volunteer who would often holiday in France for 2-3 months during the summer, so naturally they were away from their volunteering for 8-12 weeks.
I have had five or six weeks away from volunteering in the past (when staying with relatives overseas) and I can take days off whenever I like. I'll shortly be calling my manager this morning to let her know that I won't be in to work this Sunday because I was invited to a birthday party yesterday.
Volunteers also generally have the freedom to choose jobs that they want to do and to turn down jobs that they don't want. If a volunteer wishes, their only role can be pricing stock and nothing else.
I would also say that Oxfam specifically probably does not pay the market rate for their shop managers - unless the market rate is only a tiny bit above minimum wage. At Oxfam, I would argue that shop managers are significantly underpaid - they are under a lot of pressure and their responsibilities aren't necessarily feasibly completed in 35 hours per week, so many end up doing overtime which is unpaid. I recently saw an advertisement for an opening for an Oxfam shop manager near my local area. It was a 30-hour per week role and the salary they were offering was just 30p above minimum wage.