I purchased three items from the one seller just over $300 but did"nt realise about the 10% offer.

 
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What is your question? Without more details it is difficult to help you.  I cannot see anything about 10% discount on things I am searching for.  Was it an eBay discount or a Seller's discount? what were the conditions or exclusions? I don't know if you can claim it after the sale, as I said, depends who was giving it and whether they will allow it after the purchase is made.