Coffee is the lifeblood of working Americans, and even on the weekend, it's a great way to wake up and get a boost of energy. Garage sale shoppers still sleepy after being dragged from their beds on a weekend can very much appreciate a cup of coffee at the first sale they stop by. As a host, you can offer coffee and make their day, just by handing them that little cup of hot brown liquid.
Should you charge for the coffee? That's up to you, and the quality of the coffee. Oh, sure, McDonald's gets away with selling crappy coffee, but it's very cheap, and people know what they're paying for (plus it's convenient to get a filling breakfast and coffee in one spot). If you reduce the price of a cup of Mickey D's coffee down to a garage sale price, you won't be making more than a few pennies off a crap cup of joe. So, if you're offering inexpensive product on your end, you might as well hand it out for free in small doses. However, if you've bought some quality beans and are going to make fresh pots every hour, you can consider asking for a small donation.
Serve the coffee in un-coated restaurant cups, preferably of a small enough size that you won't be tearing through pots of coffee too quickly. If you use cups with wax coating, it will melt off and enter the coffee, which can be both poisonous and disgusting. (This means no Dixie cups!) There are other sample-size cups available that you can use, though some only hold enough for a tiny sip.
You probably want to give away more than a sip, especially if you're charging for it. Eight ounces is a "short" size at Starbucks coffee stores, and fast food restaurants that serve coffee have followed suit with that as their smallest size. It's enough for several large sips, so it's a good starter size. You should only consider larger sizes if you've got the resources to make large batches.
When should you offer coffee? You may consider offering it all day, as long as you consistently keep it fresh and warm, because some people just don't wake up until later in the day. However, your morning customers will be more willing to consume caffeine that those who appear closer to closing time, so you may just want to offer coffee early in the day and cut it off at, say, noon.
Condiments for coffee generally include sugar, artificial sweetners, and cream or milk. You can offer any and all of these (quite a few customers will be very grateful), but they will add the price, so make sure you factor that in and don't end up going into the red just because you decided to serve coffee.
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