Also, I just setup a personal sezzle account to trouble shoot this - I didn't have any issues...you do have to go all the way through the checkout process on the site and then let the shopping cart transfer you to sezzle - if you use a quick cart option then it doesn't work
for completeness, here's the text from the email we sent out:
Topicals: Buy One Get One 60% Off
Buy All Out Sweat or Ultra Edition Get Amino Essentials FREE (must add Amino Essentials to cart)
Save 25% on everything else (including stacks)
No Coupon Required - Discounts automatically applied at checkout - since discounts are applied at checkout automatically, these deals will not work with PayPal express checkout.
*Limited to quantities on hand, Not valid on previous purchases, Cannot be combined with any other offer, discount applied at checkout - will not work with PayPal express checkout - must add free and discounted products to cart to receive them