The best tactic to increase sales for the online garden supply store among the options provided is sending a discount code for the next purchase after each purchase.
This approach not only encourages repeat business by incentivizing customers to return but also creates a sense of value and appreciation for their current purchase. Discount codes can effectively drive additional sales and foster customer loyalty, which is crucial for ongoing success in an online retail environment.
While the other options may have their merits, they don't directly stimulate immediate sales in the same way that a post-purchase discount does.