Flat Rate Shipping allows you to incentivize your customers by offering discounted shipping when they spend a certain amount of money. For example:
Standard shipping is $9.99. Free Shipping for orders $99 or more!
This is a proven technique in encouraging your donors or customers to spend more money.
This feature needs to be thoughtfully considered. It may not work if you ship large or heavy products. However, for most apparel and books, it works great!
If you would rather charge your customers the actual shipping costs based on the weight of the shipped packages and the courier used, consider using Courier Rated shipping.