In your case, I wouldn't bother. As discussed in the previous post, marketing will increase your sales slightly, but you're not going to see a massive increase. If I got 25% extra then I'd be very happy. 50% I'd be over the Moon, and 100% more and I'd think I'd discovered the Holy Grail of marketing.
As I make a pretty decent amount of money every month from my stock sales, I'm happy to invest a bit of time and money in the hopes of achieving even that 25% increase. You're making an average of $3 a month, so a 25% increase is going to equate to an extra 75 cents a month. I don;t know of any ad service that you can use with a budget of less than $1 a month, and even if you're happy to earn $1 an hour, then 45 minutes work a month on marketing isn't going to scratch the surface.
The way that most people market their stock portfolio, at least to start with, is just by uploading their portfolio to a stock marketplace and let them bring you the traffic. Then, when they've built up their portfolio, know what sells and what doesn't, and have started to make some reasonable money... then they can consider some marketing themselves.
I mean, the big four invest several person-hours and spend millions on advertising, marketing, maintaining social media presences, blogs etc. If we're investing a few hours and $1/$10/$100 a month... we're never going to come close to the traffic that they're going to bring our portfolio. So a 25% increase is pretty good... a 100% one is amazing. Is it worth you investing a load of time and money to earn an extra $3 a month?
The best thing for you to do is to make more items and make better items. Invest time in the quality and commercial appeal of your work rather than in marketing your work. For now, at least. Look at what sells and what doesn't sell when it comes to styles and content. Take a long hard look at your content and compare it to the top sellers... what's wrong with it? How could it be better? Are your keywords any good? Could you be uploading to more and better agencies?
There are people out there who have less items than you, but make over 100,000% more a month. You need to figure out why that is, and what you need to do to fix it. Posting examples of your work here wouldn't hurt.