If the minimum for free shipping is low (like $35), I'd buy at least $35 worth. It's over 40 miles to my favorite quilt shop, so between gas and the extra I'd spend when I'm there, it would work out better.
Some places have great prices and also offer free shipping on occasion. I just ordered a bunch of FQs at a really good price and got free shipping on them. Most places is at least $5 shipping. I got a good deal.
What I "need" has little bearing on what I "must have"! LOL