They are in fact Open To The Public. People buy wedding supplies here all the time. It is not however a store that you can just browse. It's a warehouse and you will need to find what you want online before visiting this location. They do provide some computers for onsite ordering but they run a bit slow so be prepared.
It is a wholesaler. It doesn't do retail.
They are open to the public. You can't buy individual boxes. Most boxes are sold in packs of 25. You can order online and pick up at will call the next day or have them deliver to you locally for $15. Same day pickup is normally not available.
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