Rounding for cash payment

In Canada we no longer have 1 cents currency so all cash payments have to be rounded (up or down) to the nearest multiple of 5 cents. I know there’s no native support for this for WebPOS so I thought of using a product item (of 1 cent) which I can add to an order to adjust the total. Like if the original total is $4.99 then I can add the item to bring it up to $5.00.

Well, rounding up using this method works, but rounding down doesn’t because the “Need to Pay” value is calculated wrong. (Please see the attached screenshot) I’m using an item with a negative value in this case, and the value is correctly shown at the item’s price ($-0.11) and also calculated correctly for the “subtotal” ($4.75 + $-0.11 = $4.64). But then the “Need to Pay” is completely wrong, which the negative value is added as if it’s a positive value. I have no idea why the “Need to Pay” value is not just the total of “Subtotal” and “Tax” but somehow is re-calculated again and it’s wrong.

Is there a solution to this (i.e. correctly adding up the “Need to Pay” value)? Or better yet, a solution to the “rounding for cash payment” problem? As I know this doesn’t just apply to Canada but also Belgium and Switzerland as well.