@paul72 thanks for your message. Yes you are right. But the customer only sees the quantity as 5.30 printed on the invoice ... which makes no sense when multiplied with the rate.
I see what you mean - but, because of the colon separator rather than decimal point, I do read it was hrs:mins
As above, someone may come back with a simple tickbox to check/uncheck to convert to decimal but I think hrs:mins is the only option when you're using a time billing product/service. When invoicing for goods the default is decimal.
The only workaround I can suggest (other than moving away from the linked billable hours) is to alter the invoice template to show Qty (hrs:mins) in the column header to make it clearer.
Hope this helps.