You can set up a product to be priced per unit so that the user will be prompted to enter the quantity when they press the product button on the EPOS.
To set up a price by unit product, add the product as usual on your back office and set the sale price to the price of one unit (e.g. gram, or ml)
Then go to the stock tab and set the 'measurement unit' to be the unit that the price relates to.
Then go to the advanced tab of the product and scroll down to 'Other Advanced Settings'.
Switch 'Variable price product' to 'On'.
Then switch the type flag to 'Price per unit'.
The default decimal places a user will specify when entering the units is zero (whole units), but you can change that here if you would like this to be more granular (e.g. if you price in KG and customer can buy 0.45kg).
The user will now be prompted to enter a number of units when they press the product button on the EPOS.