The FBI seal, along with all other government seals, are public domain. We paid for them with our taxes. Yes, there are laws saying that you can't use the FBI seal to misrepresent yourself as an FBI agent, or acting on authority of the FBI, but you have a right to display the seal in other ways. Same as any other government seal. Wikipedia is using the seal in an educational manner. Fully protected and legal use of that seal.

I'm not looking to shoot you down, but I have some thoughts you might want to consider.

You are not going to disrupt the industry by providing a pdf for a cheap price, you are going to be working twice as hard for half the pay. Right now you may have 25 design templates ready to go, how many will you need next month?

How many hours will that take?

Are those billable hours?

You are making the design choices easy. Concentrate on that.

Instead of looking through a huge binder of printed samples, the bride to be can look at a website and get a design made simply, without the usual headaches to the designer.

Hopefully the pdf you provide will be easy to preflight and prepress, because a lot of your customers are going to end up taking it to a real printer after they see the results they get at home. That can be an additional profit center for you, if you are willing to charge what your time is worth for anything other than what gets generated by the templates.

There are not huge profit margins in most printing, even though letterpress commands high prices, the process is very labor intensive, and that wonderful Crane Lettra paper is expensive.

Don't forget that part of the final price of an invitation set is how much of a bridezilla the designer and printer had to deal with.

Finally, take a look at etsy, lot of wedding invitations there for cheap prices.

Not to be self promoting, but I am in the printing industry, digital, offset, and letterpress.

Good luck, I think you have the germ of a good idea, but you will have some refining ahead of you.