Welcome to the forum, Bru.
Drago is right.
If you order powder on line the shipping costs will be horrible.
They charge a special fee to deliver black powder, usually over $30 in addition to the regular delivery fee.
That could make your pound of 4F powder cost over $50.
Traditionally, muskets were primed with the same powder that is used to charge the barrel.
The big locks can easily light a pan full of the courser grained powder without a problem.
If you really want to buy some more black powder I suggest you spend the money for more of the same powder you intend to use for the main powder charge.
If this is 2Fg powder, you might want to try some 3Fg powder.
If you reduce the 2Fg powder load by about 10% and use it for the 3Fg powder load it will give about the same velocity and it might produce less powder fouling in the bore.