This is just a short guide - I would have thought it was stating the obvious, but here it is :

NEVER PAY RETAIL.

I consistently see people paying more than retail (incl. postage) for Dermalogica products in particular.  Why?  I don't know.  There are plenty of places online where you can buy them cheaper.  The sample sizes are super expensive per mL or per gram.

On a selfish note, paying MORE than retail drives prices up for everyone.