Thank you very much for the detailed explanation!! I appreciate! Thanks to you, I was able to understand much more:)

All good! I can qualify two meanings just a little. Off the charts - much more than expected. To be paid under the table - I wouldn't exactly say illegal, it is more avoiding tax (sales tax, income tax). A variation used here is "a cash job" meaning physical money is paid, but the government cannot trace the transaction, so no tax is paid. Both sayings are about things that aren't actually illegal until the tax return is presented to the government.