Seriously though, I does not surprise me for a second that you cannot combine offers like these. SSC are in business to make money and they would never deliberately give multiple offers to make a huge gouge in their profit.
I don't get free shipping as I am in the UK so it would not apply to me anyway, but even I (a person notoriously crap at reading small print) read the T+Cs on offers and voucher remuneration.
Do they already make a great deal of money? Of course they do. But as hard as it is to admit, we cannot really justifiable blame a company merely because we have not read the terms and conditions properly.