Speaking as a former dealer in black powder firearms and a Federal Explosives Licensee by the ATF for 35 years, an F.E.L. license normally requires a storefront business. Then, if the store is located within city limits, the city and/or the fire department can add fees and restrictions. The ATF has regulations on the size and construction of a magazine with locks that cannot be cut with bolt cutters. The locks have to have a minimum number of pin tumblers, etc., etc. Your business insurance agent may have reservations about carrying it.
The Fire Marshal in Fayetteville, NC gave me an ultimatum, close down or spend the money to have my 300 pound steel-plate back door reversed to swing out instead of swing in. Putting the hinges on the outside would encourage a break-in. But she would not budge from her stand. My attorney, my landlord, and I had to make our appeal to the NC State Fire Marshal in Raleigh, and three months and $ 5,000 later, I got to keep the hinges on the inside.
I recommend Powder, Inc out of Clarksville AR. A minimum case is 5 lbs, next is 10 lbs, last is 25 lbs. The beauty of ordering is one can mix grades and brands to get the lowest cost per pound, including HazMat & shipping. For instance, Swiss powder, favored by national & international competition shooters, is $31.85 in a five pound case and $23.05 in a 25 pound case. So, you get a bunch of friends together to order a 25 lb case, and collectively, they order 23 pounds of GOEX @ $16.20/lb and you throw in your order of two pounds of Swiss 3FG @ $23.05.
Order a ten pound case of GOEX @ $20.05 = $200.50. Five pounds of GOEX will be $127.50, delivered. All the stated prices include shipping & HazMat charges. With the current mood of Washington, having more than two pounds of powder cannot hurt you financially. Hope this helps.