I am looking to have this plugin installed on our CS-Cart multi vendor (latest version)
What would the cost be - and will this affect site speed?
CS-Cart has this feature in the 'E-mail marketing' addon since v4.2.1 version.
Great thank you 
Please find more details here:
CS-Cart Documentation — CS-Cart 4.15.x documentation
CS-Cart Documentation — CS-Cart 4.15.x documentation
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Great thank you
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don’t forget to change source from newsletter to mailchimp in the neesletter block to avoid 404 
Dear Friends,
We have developed the MailChimp Advanced add-on. It allows to create unlimited mailing lists, eye-catching and user-friendly sign-up forms and pop-ups.