I like to KISS. But is this landing page too bare?

I like it, it actually - I think having the entire service presented as useable before anyone even gets to the FAQ is cool, it's so simple it requires no further explanation.

I have 2 additional thoughts - I think you should offer a kind of letterhead capability and possibly themed envelopes and paper unless you want it to stay totally bare bones. You will be leaving money on the table though because it's almost too minimal. Let say you skip the paper and envelope choices, not a big deal, but a header option might be nice like "From the Desk of John Smith" maybe with contact info under it and date added.

a "see what it will look like" feature showing the letter and the envelope might be very cool - it kind of closes the loop on the "what are they gonna receive?" thought the user might be having, this should be shown or be an option before payment - like at any time while it's being written I can see what it looks like.

People might want to use this for legal reasons - you might even offer an upcharge for sending it as a registered letter. That gives it a whole different purpose and puts it into a different use case world - yours look like it's aimed at sending personal letters. a simple check box that ups the price is all that would impact the presentation so it would be minimal.

Alternately you could make a twin version of this called "registeredletter.us" that offers more editing controls and does an API thing with the post office with tracking etc. People need to send registered letters but they have to write them, print them, go to the post office, etc. Think how much friction this would remove! I would first check to see if that already exists though...

The letterhead controls, the registered letter, and the "see what it looks like" controls would be minimal adds - this doesn't even offer font size, font choice, centering, bold, etc. It's basically a typewriter LOL. You could just say "scroll down" and dynamically be generating the "see what it looks like" as people type.

You should also include "generated by sincerelyme.us" to add a touch of virality, next to your barcode in the footer of the letter.

Last thought - you don't store the addresses, it can be paid for with a visa gift card, it's mailed from somewhere nearby or somewhere away from where the actual sender is so what's to stop someone from using it for stalking and threats, etc. You may want to keep a paper trail on this starting with the IP address of the sender, time of day, the entire message, etc. because once it's used for a bad reason you're going to be holding a subpoena from some court somewhere. You can choose to not even bring the subject up in your FAQ, store the data for a year where it isn't ever accessed and then delete it later.

Nope, I don’t think so. If you’re in doubt, just do some A/B testing to see what works (and what not) ;-)