The fees specified in §§ 160.201 and 160.202 apply to the routine field inspection and usual laboratory work incident to the certification of commodities covered by those sections. Should additional work be required to provide special information desired by the person requesting service, or should it be necessary for an inspector to make a special trip or to deviate from his regular schedule of travel, or should the fees prescribed in §§ 160.201 and 160.202 otherwise be insufficient to defray the cost to the Government for rendering such service, then the person requesting the service shall pay, in lieu of the prescribed fees, an amount computed by the Department as sufficient to defray the total cost thereof, including allowances for time spent in collecting and preparing samples obtaining identification records, traveling, performing laboratory tests or other necessary work, and also any expense incurred for authorized transportation and subsistence of the inspector or analyst while in travel status. The charge for time so spent shall be computed at the rate of $17.80 per hour for laboratory and field inspection work. The overtime rate for services performed outside the inspector's regularly scheduled tour of duty shall be $21.30. The rate of $26.70 shall be charged for work performed on Sundays or holidays.