You gals are all very lucky ... the folks sending the bags to you knew what they were doing.
I've answered this question about Customs & Duties many times, and I always tell folks that they should review the US Customs website to get all the facts on Customs & Duties.
The duties assessed are based on the "value" as filled out by the sender. By default, you are allowed to bring in $800 (per person) worth of goods without any duties being assessed. Therefore, if your sender listed the item as having a value less than $800, that would be why you didn't have to pay anything. If the sender puts an amount greater than $800, then you are assessed a flat rate percentage.
As the US Government has been focusing more on terrorist activities, I have found them to be much more amenable - many times either indicating no duty or assessing a very low percentage (1%). Either way, the sender (anything outside the US) must fill out the customs forms or the parcel will sit in customs and they will contact either you or the seller to provide the necessary paperwork. When I ordered a bag from Browns (London), it sat in customs for 1 month because Browns had not provided the paperwork. When they finally did, the put the 'real' value so I ended up having to pay duties to boot!!! Needless to say, I wasn't too thrilled ...