Knowledge is power. You feel more confident when you are armed with right information, and you'd feel less embarrassed. Both Priceline and Expedia own other popular travel sites. You may think that you are visiting their competitor's site but it is likely to be one of their sites. For example, Priceline owns Agoda, Booking, and Kayak, while Expedia owns Hotels, Orbitz, and Travelocity.
When it comes to flight searchs, don't rely on Expedia or Priceline alone. Try the same search on Google Flights or Matrix Airfare Search because the other sites may not show some flights. Don't get excited when you find a very low airfare on Google Flights. Some flights are just phantom flights. When you try to buy it, you will be told that it doesn't exist.
If your credit card charges foreign transaction fee, you may incur the fee even when you complete the transaction in your home country because the payee is located in a foreign country. The fee doesn't show up on the payee's bill because it is added by your credit card issuer. Find out before you click on the buy button. You also need to be aware that some foreign hotels charge you their fees in their local currency at the time of your check-in, which means you could pay more in your currency because your currency is worth less than when you booked the hotel. Read the fine print before you click.