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Generally agree with the comments - links between two of your own sites shouldn't be a huge problem. Google could devalue them a bit, if they see the connection, but devalued isn't penalized. As Philipp said, if you also put contextual links to the webshop in the blog, it could be that the banners keep those links from counting - Google tends to disregard the 2nd, 3rd, etc. link to Page B from any given Page A. If the contextual links are specific, deep pages, though, you should still get value.