How to advertise an inexpensive product?

Question

Internet advertising

Good evening.
I want to create an online store and sell my pdf books for children. Books in two languages ​​- English and Russian. The price has not yet been finalized, but I think it will be ~ $ 3 per book.
Question: And now I'm thinking, how will my site be found?
After all, launching advertising on Google, for example, will not work, since advertising will eat up all the income (if not more). Gadget app makers have the same problem because they sell at the same price, if not cheaper, how do they advertise their product?
I found only three options for myself, but I would like to know the opinion of more experienced people, otherwise my options are very long and unlikely.
1. SEO content, but this is more of an investment for the future.
2. Go to different forums and blogs where similar books are discussed and write about my books. I have a similar experience in writing comments, and according to my statistics, 10 comments remain from 100 written comments, and traffic from them is at best 20 people per month. To do this, you need to write for three days from morning to evening in order to find a site and leave a comment. Yes, and there are probably about 1000 thematic places, so in 30 days the sites will end and I will have 100 moderated comments with ~ 200 entered per month.
3. Word of mouth, but before that you still need to live to gain a large number of buyers.

Answer 1

You can try to lay out books on liters and analogues.
There will be sales, but without PR they are extremely small.
smm:
Posts both paid and free in thematic groups in vk/classmates.
If the books have cool illustrations and there are a lot of them (+ new books are coming out) - instagram

Answer 2

You can run a one-cent ad on Google, your niche is likely to have low competition, so it might go. You can use teaser ads. In general, I agree with the previous responder, SMM is the most for you.