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Can I change the look and feel of the calculator?

Yes, you can edit the code after "Edit the following to change the look and feel of this calculator" in love-calculator-wp-widget.php to change the look and feel. You need to have basic HTML understanding to change it correctly.

Can I put this calculator to both the sidebar and the post?

No, please put it to either the sidebar or the post, but not both. If put into both places, the calculator will generate javascript error.