I saw a poster that said you get a discount on beer if you “friend” the shop on social media, but when I ordered it, I was charged the regular price.

In general, discounts are offered for specific items such as a small-size draft beer. So, if you order a draft beer that is medium size, the discount will not apply. Check with the shop’s staff to be sure that the beer you ordered is the item that is discounted. Or, if the wording on a poster could be misunderstood, it might be good to point that out to the staff and try to negotiate. At such a time, communicate that it might be a good idea to fix the poster to make it clearer and avoid a similar misunderstanding in the future.