Advice Please! Greenstuff Bulk Buying

I'm looking for advice on buying greenstuff online. I found this website and a bit of research seems to show that it this company that produces the greenstuff for Games Workshop. That is why I wanted to buy it elsewhere: It's €9 for 8 inches of the stuff from Games Workshop, €11 for 36 inches from this crowd!!

I'm planning on buying tubes of blue and yellow so none of it has begun to set.

At the same time I need new knife blades and thought I'd kill two birds with one stone. So does anyone know what size blades the GW knife uses?

The main purpose of this post is to see if anyone knows if this website is trustworthy. It doesn't accept paypal so I have to be absolutely sure about it. Advice please!