I was wondering what the general consensus is on using a credit card statement as a receipt. Only for purchases that are clearly for a specific vendor and/or related-expense. Not just a general Wal-mart transaction used on the taxes.
Credit card statement is questionable. It may not fly during audit since from the statement you cannot learn what the purchase was for and what items were included. When small amounts involved and the nature of the expense is clear (e.g.: you're claiming a meal expense and you have a credit card statement showing "Starbucks") it will probably be fine. But if the expense is reasonably large (>$25, I'd say) and the nature of the expense is not evident (e.g.: looking at your other question - paying $1500 to the university bursar, claimed as tuition expense) - I expect it to be disallowed.
Generally, in order to justify an expense, you're expected to have an itemized invoice/receipt showing payment for specific items which are reasonable and necessary for the matter at hand (i.e.: a receipt from the bursar showing "Course XYZ-101 $150 per unit, 10 units, $1500 charged" will be a sufficient justification for tuition deduction of $1500).
For invoice - you can show the credit card statement with matching amount to prove the invoice was indeed paid.