Updated February 2017: According to the Bolivia US Embassy website, the Bolivia entry requirements listed below is still accurate.
After an adventurous time exploring Peru, I took the Bolivia Hop bus to cross the border to Bolivia. Little did I know, as an American, I needed about 8 different things to be allowed to enter.
Caught off guard (I am always over-prepared!) and frustrated that I didn't research the information beforehand, I rushed to get all of my paperwork at the last minute. Though the regulations are rumored to change frequently, as of June 2015, here is what you will need to cross the border from Peru to Bolivia if you're American. I'll update this page if I hear any updates.
Two passport-sized photos
I had no idea about this but was lucky to learn that right next to the immigration office on the Bolivian side of the border, a local couple owns a small storefront selling snacks and making photocopies. Recently, this couple purchased a digital camera and printer and can provide small photos for the visa requirement. I crossed the border, went to the back room in the store, and got my picture taken against a white sheet and window. It definitely was not high quality but it was good enough to submit to the office next door.
Two copies of your passport
Thankfully I carried these around with me anyway. If you don't have passport copies, make sure you at least have a copy in your email drafts so you can print it in an internet cafe. The aforementioned store next to immigration also has a copy machine.
One copy of your bank statement
Why? I don't know. But you need it.
I hated this the most because I had to run into an Internet cafe/copy shop at 9:15pm (my bus left at 9:30pm) and pretty much cried asking the shop owner not to close so I could log into my online banking account. The whole transaction felt last minute and uneasy. Apparently the U.S. does this to them when they try to visit our country so maybe it's a payback thing.
A day-to-day printed itinerary
Again, I don't know why. I DO know that I have never planned a full vacation itinerary in my life! I made the itinerary up while at the Internet cafe and was sure to throw in the name of a hostel chain or Airbnb address.
Be sure to have the complete address of where you are staying.
Copy of your yellow fever vaccine certificate
I made this copy at the store next to the office.
Proof of onward travel
I printed my pre-purchased bus ticket as proof of exit. If you don't have one, buy the cheapest bus ticket you can find, or purchase a fully refundable ticket before crossing the border so you can cancel once you're in the country.
$55 cash fee
Self-explanatory. Except if you don't have the proper paperwork you pay $135 (visa application). Note that although U.S. currency is accepted, no one in Bolivia accepts dollars that are ripped or wrinkled. Apparently the bank gives them a low exchange rate if they are not crisp, new bills.
If you do not have the Visa paperwork (Bolivia Hop provided it to us) expect to pay $135.
Three pages of visa/customs paperwork
When I was at the immigration office I didn't see the forms laying out so they're either behind the desk and you can get them after paying the higher fee, or you need to stop by a tour company office to get it beforehand. I took the Bolivia Hop bus and they supplied it for me.
I hope this helps! I wish I'd had known! xx