teespring.com
is telling your visitors' browsers that they should not display it in a frame.
You have four options:
- Talk to whomever runs
teespring.com
and convince them not to do that - Just link to them instead of trying to shove them into a frame
- Don't point your visitors to them at all
- Proxy their site and point the iframe at your proxy (this may step into copyright infringement territory)