That's quite a bit for shipping. It's basically what people sell the plants for on ebay with free shipping.
If your intention is to help people get a little frog bit (which is what it would appear by the free posting). You could offer a smaller amount you can put in a envelope lined with bubble wrap and in a ziplock sandwich bag and some wet paper towels in and ship it for $3- $5.00 (that's about what it should cost but you will have to drop the post office and have them weigh the envelope and get a price)
Here's some info on how to ship it (sorry if you know this already)
Yes it a bit it is because the post office are so far away from me so I have drive there so I have to pay for the gas but you will get a small 2 day shipping small box full of frogbit someone even told me is cover all 3 of there tank (7.5g gallon altogether and there was a still a bit left
You needn't travel to the post office at all if you're sending via Priority mail (the 2-day service). USPS will pick the package up for free when they deliver your regular mail. This will allow you to send a small flat-rate box for $6 without any extra charge.
From the USPS website:
"Your carrier can pick up your shipment free during regular mail delivery if you're using an expedited service like Priority Mail®, Priority Mail Express®, Global Express Guaranteed®, or Merchandise Return services."