You cannot sell a quantity that is not an integer. That means you can sell 1, 2, 3, or 311 of something. Or 420. Or 999. But no, not 0.1, 0.2 or 16.897 of anything.
If you want to sell 0.1 of something you have to convert that to 1. So if someone puts 1 in their cart, they are buying 0.1. A 0.9 is 9.
This is similar to a unit price. When you need to sell a random amount of something, say the length and width of a picture frame, you collect the total number of inches and then conver to an integer and sell as $/per inch.
So whatever you sell... you have to sell it in units of integer value. In your case... just price 0.1 as 1 etc... so really this exercise for you is one of pricing and words to explain things to customers, not collecting a quantity of 0.1, 0.2 or 5.4 which will never work with Shopify.