Customers are forced to change the product amount in cart. (Selling per sales unit only)

[Anne-Marieke]

Hello everyone,

A old colleague of mine set up our webshop on which we sell B2B, so we are only selling products per sales unit. He did this using the Vendor part for the sales amount, see image below. In this case this garlic sauce is only sold per 12 bottles. You can then only add them per 12 to your cart as well using + and - buttons. If I add for instance a '7' here, the product is only possible to add per 7 to your cart. 

In almost all orders this works without any problems. But sometimes I get a call from a customers that they are forced to change the amount of the product to weird amounts. The text they get shown is translated to English: 'Please enter a valid value. The two closest valid values are 0 and 16.'. Customer then changed the product to 16 (which should actually not be possible, since this product is only sold per 12), but received a second notification 'Please enter a valid value. The two closest valid values are 12 and 24.' If the customer changes the amount again to either 12 of 24, another product pops up in his cart with the same error message. If the customer has a lot of items in his cart, this is a very tedious problem. See a screenshot from a customer below. This customer was using Google Chrome on a Windows computer. 

I have no idea what causes this problem. Especially since it has happened only like 5-10 times on 195 orders so far.
Is there anyone that recognises this problem or know by which this is caused? 

The webshop is www.bickerykerst.nl
You have to make an account to add things to your cart. Let me know if you do, I will approve your account so you can use it.  

Also let me know if you need more information. 

[Anne-Marieke]

No one that recognises this bug?

[Anne-Marieke]

Still looking for a solution if anyone recognises this issue.