I really don't understand the one star reviews unless the people who are using the site have really not understood the way that it works.
I have been using the site since 2013 and have had a few hiccups, such as poor quality merchandise (not really the fault of Zulily), mislabelled merchandise (tried to return as inappropriate, told to keep it, item was fully refunded and a $10 courtesy payment for my inconvenience), and cancelled items (running total of three items, each refunded and an inconvenience courtesy payment made).
The easiest way of getting the minimum shipping is to wait until Friday to order your items (which does mean you can miss some deals), purchase a single 'light' item, so you pay the minimum shipping charge (I believe this is currently $5.95), and then your shipping is free all weekend! They also have some deals where they have a week of getting free shipping with a Visa credit card as well.
Like with all things, if you shop cleverly, buy brands that you absolutely know your sizing, are happy to purchase branded items which maybe seconds or from several seasons ago, and are happy to wait for your items (though there are also plenty of items that can 'ship sooner' as well), then Zulily is the place for you to go. Otherwise, spend more money and buy from other online stores.