@TigerKingQueen
Your OP was (aibu) To not know how to earn more or progress?
Hopefully we've convinced you that Part 1 - re-branding of your skills - has shown that you have lots to offer and plenty of potential to both earn more and to progress.
Now for Part Two - do you like the company that you work for? In particular, do you get on with the owner, are you trusted (which you must be as you are the manager one week in four) and would they be open to you suggesting ways in which you could progress?
If the answer is no, then the next action is pretty clear - get on the internet and start looking for something else.
If the answer is yes, or even a strong 'maybe,' then you can take control of your next steps in order to progress.
Assuming that the company is an online retailer or wholesaler.
Are there different categories of products - for example, an online clothes shop might have menswear, womenswear, childrenswear etc. in which case could you ask to be in charge of one particular category? This would mean taking over some areas that you might not currently do, for example doing more of the purchasing or sourcing rather than just adding products that the owner has found. Is there a customer segment or product segment that you could add to the current range (e.g. stock more for teenagers, or expand the range of lingerie or maternity wear) that you could be responsible for?
Obviously, you can adapt this to whichever business you work in.
The benefits are that you expand your skills and experience, make yourself more knowledgeable and more valuable to the company, (hopefully) increase the turnover and profit, take some of the weight off the owner's shoulders etc. >>> all of which puts you in a position to negotiate for more reward. You probably cannot easily ask directly for a, say, £4,000 pay rise, but you could try phrasing it differently:
£1k for taking on more responsibility, £1k bonus if you add £x to the turnover within 6 months, £2k bonus if you add £x to the profits in category X, and so on.
If the owner says yes, then the rest is up to you - but remember to ask for support from the boss in areas that are new to you.
If they say no, then at least you know where you stand regarding earning more or progressing in the company.