Should I ask for more money?

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So used to work at Walgreens, at the front register. I made 8.50 per hour, hated it.

Any customer that came in that looked like he did work, like carpentry, painting, plumbing ect, well I asked what he did and after he told me I'd ask him if he needed help. Had two different guys say yeah, even exchanged numbers and that turned into nothing, they'd just say next week so on and so forth.

Finally one of the customers said yes, I worked a day with him, made sure he had enough work, was legit ect.

He pays me $13 cash (under the table no taxes) which is a big raise to me. My brother , however, keeps telling me to find a union, that I'm worth more, that this guy should be paying me $18 per hour if he's paying me cash which is what it'd cost him if he paid me $13 properly, with taxes, workers comp, and unemployment insurance.

Should I ask my employer for more money? Oh, I refinish and repair decks and piers btw. I'm still learning also, he wants to get me to the point where I can do everything on my own, which will be soon.