Forward your supplier's emails to one dedicated address. Each invoice is compared with the price list you were given, line by line, and the differences come back by email. No login, no complex setup.
One confirmation email. No password, no card.
What comes back: the invoice against the price list in force, the order against the invoice, and the new price list against the previous one. Every reply says what it read, what it compared against, and which lines it could not match — with their amounts.