This is similar to (but not the same as) samples supplied free of charge discussed in a previous blog. The issues around commercial invoices versus pro-forma invoices also discussed in a previous blog apply here.
The aspect of samples has to do with valuation, i.e. the value of the goods for Customs purposes. Again, this affects the rate of duty and Vat. Should one rate samples as “free” or “zero” on the commercial invoice, or should there be a value even though they are being supplied “free of charge”?