You know OP, you're stuck in a pretty nasty situation when it comes to teaching these people of Bitcoin and the benefits that it entails. For starters, you can't really expect people who can't even read nor write to know or have a care about anything that involves numbers. To them, money that goes to their hands is more powerful than any thingamajig you offer them, you can't convince them otherwise, plus they are already old so no novel stuff will effectively go across their heads. As for the youngsters, I see that these people are influenced by their elders and parents, or the case could also be that they have priorities set of their own, that are completely off from you teaching them about bitcoin and its wonders.
Why are you jumping into a conclusion that the people @OP dealing are illiterate? Not because they don't want to use Bitcoin as payment, they don't know how to read or write at all. I believe @OP's method is wrong in convincing his buyer to use BTC. Or it is also possible that these people are so comfortable in using fiat money that they ignore any other options. Bottom line, @OP need to think of an effective approach to encourage his client to use BTC.
What I really suggest is that you pool whoever's willing to listen. Hold a poll or something in the town hall, have people list their names if they wanted to attend a seminar that would help them be more aware of bitcoin. I see that as the only way to really get people who will listen.
That's a lot of work while @OP can go on with his business without these people paying in Bitcoins. But if OP really want to advertise Bitcoin and make people pay with it, I would go with price adjustment when paying in Bitcoin just like one of the earlier reply suggested. I think that is enough to catch the interest of @OP's client.
In my opinion, rather than immediately adopting it, it is far more important to provide an understanding of what bitcoin is and what are the advantages of adopting it. Adoption issues are never going to work out in a day unless you are somewhere very familiar with bitcoin. Bitcoin is a new type of currency, so it is not easy to get someone who previously only knew fiat to agree directly with bitcoin. However, this requires time and effort, including providing information about bitcoin among people who find it difficult to accept its price volatility.
I think this won't work. Most farmer wanted an instant result or benefit, and @OP must make a situation where his client have to pay in Bitcoin one of which is giving them advantage/privilege if they pay in Bitcoin.