I went to the post office to mail a letter.

我去邮局寄了一封信。

I went to China Post to mail a letter today.

我今天去中国邮政寄了一封信。

In 1799, it would cost so much to mail a letter-- I don't know exactly, something like $10 or $20 to mail a letter at today's rate-- and it would take a long time get there.

在1799年，寄封信需要花费-,我也不清楚具体的数字,大概相当于现在的10到20块钱-,而且还要花很久才能送达。