The instalment fee is calculated at 1.1% of the transaction value at checkout.
For instance, a $1000 with a 3 month interest-free period. If you choose to extend it to 6 months interest free, here's what happens:
1.1% of $1,000 is $11, this fee is then multiplied by the 3-month extension period, giving a total of $33.
The $33 total extension fee will be split evenly across all instalments meaning you'll pay an additional $5.50 per monthly instalment repayment.
It is important to note that if you decide to opt out of extended instalment repayments early, you will not be eligible for a refund on any instalment fees that have already been charged. However, any upcoming fees will be waived.