封筒 and 切手: inserting letters and attaching stamps
Distinguish mailing supplies and the targets of inserting and attaching.
An envelope for contents
封筒 (ふうとう) means envelope. 「封筒に手紙を入れました」 means putting a letter in an envelope.
「封筒を開けました」 means opening an envelope. It does not establish that the enclosed letter was read.
A stamp to attach
切手 (きって) means postage stamp. 「封筒に切手を貼りました」 means attaching a stamp to an envelope.
「切手を二枚買いました」 says two stamps were bought. 二枚 gives their count, not their value.
Checking modifiers and targets
「切手を貼った封筒」 identifies an envelope with a stamp attached. 貼った describes the state of the following envelope.
「切手ではなく、封筒を買いました」 corrects the purchased item to envelopes, not stamps.
Does 「切手を二枚買いました」 give the total amount spent?
Show the answer and why
No. It counts the stamps but does not specify their individual value.