You don't. I get free shipping from them all the time. You have a choice in shipping methods. Their least expensive method is free if the purchase is over $150, and all of their more expensive options are greatly reduced.

Certain items that they are no longer able to reorder come from their "Vintage" warehouse which is separate from their main warehouse. If you order an item from their vintage warehouse with other regularly restocked items, they charge you separately and you will have to pay shipping on both the vintage items and the regular items. If you order $150+ worth of regular or vintage items you get free shipping on only those items, and must pay shipping on the items from the warehouse that did not total $150 or more. It's a bit silly, but it's how they do things, and you can verify your shipping costs before you commit to placing the order, so if you're not going to spend $150 or more on their vintage items, you may as well shop eBay first for any items that may be available at lower price. I recommend Transformerland.com if you're looking for something that is out of production; you'll still pay shipping, but they have pretty good pricing, great communication, and reliable service, if you don't want to try your luck on eBay.

Free shipping at TFsource gives you the cheapest, worst possible shipping service around. I always had to pay for UPS or Usps just to be sure there were fewer hassles with my orders. However when stacking orders took place I had to jump through circles to get them to not use their cheapest service because they felt they had a right to refuse my extra dollars seeing it was still cheaper for them. I would send e-mail after e-mail and call them and leave messages and they would actually tell me to not worry they will send it with my shipping preference. Sadly this was a lie most of the time and it really says a lot about how they operate.

Did you make sure you had selected the lowest cost shipping method? If not, then you got pro-rated shipping.