Postage for a letter from the United States to Spain is determined by weight, from $1.15 (1 ounce) to $4.11 (3.5 ounces), based upon 2015 rates. Large envelopes, which include rigid or non-rectangular items, vary from $2.29 (1 ounce) to $35.08 (64 ounces).
The U.S. Postal Service price list is available on USPS.com. The international price lists divide the countries into groups; Spain is in price group 5. Country-specific restrictions and requirements for Spain are also available from the online USPS international mail manual. For example, business documents do not require a customs form, but merchandise samples and advertising materials do.