I am not trying to patronise you. Its just when you make such a bold statement
" Now as we are already PCI compliant there is actually no requirement for our store owners to provide their own PCI-DSS compliance."
That I spoke out. Now if you had said that your customers do not have to worry about PA-DSS approved payment applications, then you would be correct. (although why 3rd party hosts like yourself who provide carts as a service get exemption is a different matter).
Your post, uncorrected, would seem to imply that your customers do not need to worry about PCI compliance. That simply being your customers means that they are compliant. It is possible that you did not mean to give this impression.
All your customers do need to achieve PCI compliance. I refer you to
https://www.pcisecuritystandards.org/pdfs/pciscc_ten_common_myths.pdf
and
https://www.pcisecuritystandards.org/pdfs/pcissc_getting_started_with_pcidss.pdf
Now I am sure that your customers, if they have their own merchant numbers, will probably fall under SAQ validation 1, namely "Card-not-present (e-commerce or mail/telephone-order) merchants, all cardholder data functions outsourced. Does not apply to face-to-face merchants". They will have to then fill in the Self Assesment Questionaire A, and adhere to the principals and procedures involved.
This is a relatively painless process, BUT it has to be done.
I know that you could point out in the first paragraph of the PCI getting started document it says "The standard includes 12 requirements for any business that stores, processes or transmits payment cardholder data." Seems to exempt your customers, as they never store, process or transmit cardholder data. BUT If you look at the PCI overview document
https://www.pcisecuritystandards.org/pdfs/pcissc_overview.pdf
Half way down the first page, on the left under the heading "PCI Standards Include :" and the sub-heading "PCI Data Security Standard:" it clearly states
"If your business accepts or processes payment cards, it must comply with the PCI DSS."
So I have no intention to be patronising. I am simply trying to stop misconceptions.