My wife signed up for a 0% balance transfer offer with BarclayCard (UK), and transferred around £1000. The 0% interest is always listed as a line item on the monthly statement.
Each month she pays off £200, but may also make some purchases that month. Some of these purchases are refunded - sometimes not within the period covered by the statement, but always within the payment period (3-4 weeks after the statement date). Non-refunded purchases are always paid off within the payment period.
However, she is often charged an amount of standard balance interest that seems to correlate with the value of the refunded items. Is this correct?
As an example, the statement period runs from the 1st Jan to 31st, in which £450.00 of purchases were made. £400 of this is refunded after the 31st, but within the payment period. The other £50 is paid off within the payment period along with the £200, making a total payment of £250. In the next month's statement there is an interest line for £5.52 at 1.38%, which matches the £400 refund.