There is no problem with the plan itself.
The price itself is not high, but shipping costs extra.
one's own body; oneself
Rare as a modern independent noun meaning one's own body or self; learners will usually encounter 自体 in the suffix-like 'itself' sense instead.
In older writing, 自体 can refer to oneself or one's own body.
originally; naturally; by nature; from the start
Rare adverbial use meaning that something is so from the beginning or by its nature; in ordinary modern Japanese, もともと, 本来, or 生まれつき is usually clearer.
When 自体 is used as an adverb, it can rarely mean 'originally' or 'from the start.'
The price itself isn't high, but the shipping fee is.
I think the plan itself was unreasonable.
Some scholars doubt the theory itself.
そのもの is a more emphatic way to say 'the very thing itself,' whereas 自体 is the neutral, understated suffix-like form.
もともと is the everyday adverb for 'originally,' while the same meaning in 自体 is rare and mostly literary.
Standard kanji spelling for じたい. It is commonly used after a noun as in 問題自体, while the older independent-noun and adverbial uses are much less common.
A Sino-Japanese compound written with 自, 'self/own,' and 体, 'body, form, substance.' This component meaning fits both the common 'itself' use and the rarer 'one's own body' sense; the exact historical development of the adverbial use is uncertain.