I may buy = I will possibly buy, but I will have the choice. I don’t have to buy it and I may not really need to buy it.
I may have to buy = If I want to have enough of this, then it is possible that I will need to buy some more. I won’t have a choice because I will have run out.
I like this bread. I may buy some more when I next visit the market. (choice)
I’m not sure whether I have enough bread left to last all day tomorrow. I may have to buy some more in the morning. (no choice if I don’t want to run out)
‘May’ as a modal often expresses ‘possibility’ as in: It may rain this afternoon. We are not sure because we don’t know.
‘Have to’ suggests ‘necessity’ as in: I have to take an umbrella with me because it is raining.
If you join those two sentences together, it becomes: As it may rain this afternoon, I may have to take an umbrella with me. In that sentence the necessity (have to) is limited to a possibility (may).