I placed an order for two types of filters. 8 units in total.
I made an error on the size for one group, which was designated as 'custom filters'.
I tried to correct/cancel the order a few hours of placement.
The initial response was because they were custom I could not cancel the order.
After a couple of email exchanges, a hero in the Filters Fast team (Sam) was able to cancel the order, even though they had already been shipped.
Thanks very satisfied customer