I think you do not need to this 8% to tax office.
In the case, the amount of your taxable sales ( kazei uriage daka ) of consumption tax is less than 10 million yen, you do not need to pay consumption tax to tax office.
Even if you charge 10,000 yen + 800 yen ( consumption tax ) to your client, you do not need to pay 800 yen to tax office.
You may think it is strange.
But,when you purchase a pen to conduct your job, you paid consumption tax.
As for withholding tax of 10.21%.
In the case a business person operates specific kind of job ( translation = hon yaku, judicial scrivener, etc ), when her client pays fee to her, the client needs to withdraw 10.21 % of her fee.
In the case, the amount of her fee is 100,000 yen, her client withholds 10210 yen.
Then the client pays 10210 yen to tax office.
Then, in the case rate of her income tax etc is less than 10.21 %, if she files tax return to tax office, a part of withholding tax is return to her.
If her taxable income is small, rate of her income tax etc is 10.21% .
So, she can receive money from tax office.
You can find information of rate of income tax and withholding tax in this website.
http://japanhousesaitama.lawyerjapanese.com/saitama/