Don't worry about your posts not appearing immediately, Kayser. Mine don't, either. Some threads are monitored by the board admins more closely than others - they probably get more spam on these threads. It serves to keep this forum spam-free (Which is greatly appreciated, Admins.)
Re: your question - I think a sign - or several signs - in the store as you suggest, would be great. Sure, there'll always be someone who finds it pushy but I don't think most will. Like everything in marketing, test, test, test. Put one or two signs up and after a while start asking customers if they find them acceptable. They'll tell you. If you get too many saying they find them unacceptable (for whatever reason) take them down, but my guess is they won't say that. Some may even thank you for the reminder.
You must monitor your FB page, Yelp account, etc. to see if the signs are working. Test the wording, check the results. Change the number of signs or the location of the signs, check the results. Change the color, check the results. You will eventually find the best format.
In fact, I always recommend to my clients that they start a Promotions Journal, in which they write down details of everything they do to promote their business - their advertising, their promotional campaigns, their publicity, etc. and the details of each change. Plus record the results. That Promotions Journal, after some time, can become an excellent reference and a tremendously valuable record of your business growth.