Do I need to have a US travel license to sell retreat packages to my customers?

if you are interested in being a tour guide in the United States, you might be required to earn professional licensing, depending on which city or area you want to work in. If you plan on being a guide in any of the following cities, you will need to get licensed first:

Washington, D.C.
New York, NY
Savannah, GA
Charleston, SC
Gettysburg Battlefield, PA
Vicksburg Battlefield, MI
New Orleans, LA

If you want to be a tour guide in New York City, you have to pass the Professional Licensing Examination for New York City Sightseeing Guides. The test consists of 150 questions, and you need to answer 97 correctly. The questions cover topics such as New York transportation, history and architectural landmarks.

Becoming a tour guide in Washington D.C. requires submitting an application, which includes completion of the Professional Sightseeing Tour Guide Examination. You'll also need to be able to communicate in English and have a clear criminal history over the last five years.
Source: learn.org

Probably, but it depends on the state(s) in which your customers are located. Travel agent licensure/authorization in the US is governed by state law. In California, for example, the California Attorney General's Office administers the Seller of Travel Program: https://oag.ca.gov/travel

Start by figuring out where your customers are located, then you can research each state's law. Here's a starting point: http://travellaw.com/page/travel-law-faq

In the US, there are 5 states that have Seller of Travel Laws, California, Iowa , Hawaii Washington and Florida. , i.e. a travel agent license. It is always good to recheck though as states could change their rules as well to try and cash in on extra sources of revenue .