If you tell them you've been in Taiwan a long time and that they should not withhold more than 6%, they will usually comply, but it's up to their accountant, generally; some will book it without withholding, and some will put it in an honorarium category that is not taxable income up to a certain amount. Have a photocopy of your passport name page and ARC ready (or a scan you can send by email) for such clients. Some will also want a copy of the name page of your bank passbook if they are paying by ATM transfer. Just comply with whatever their accountant requires, including signing a receipt in advance of actually receiving the money (this is common here at universities and research institutes).
Pleasure is spread through the earth In stray gifts to be claimed by whoever shall find. —William Wordsworth