The weight covered by basic postage for a letter depends on the destination and the type of postage used. However, assuming that the basic postage covers a letter weighing up to 1 ounce (28.35 grams) and that Emma's letter was mailed within the US, we can calculate the weight difference as follows:
1 ounce = 28.35 grams
Emma's letter weighs 12.92 grams, which is less than 1 ounce.
The weight difference between Emma's letter and a letter covered by basic postage is:
28.35 grams - 12.92 grams = 15.43 grams
Therefore, Emma's letter is 15.43 grams lighter than a letter covered by basic postage.