What are the best...

[Eric72]

...holders for sending cards to SGC?

I'm planning to ship them some cards for grading. Their website notes that semi-rigid would be preferred. However, there seem to be quite a few brands/styles from which to choose.

[brewing]

I don't think you can go wrong with Card Saver 1 size. Not sure what the Ultra-Pro equivalent is, but I've used those in the past.