"Cash Cover" / Prepaid cover
To ensure that the necessary postage had been paid, China's postal authorities laid down that all International Mail Matter had to bear some indication, that the item being handled had been dispatched from an Imperial Post Office. So , as far as the application of the relevant postage stamps were concerned, this correspondence can be classified into two distinct groups:-
    a) Mail presented to the Imperial Post Office of dispatch, without postage stamps. For such mail, Postal Instruction No. 4 laid down, that the person presenting an item of mail for dispatch must purchase and apply those stamps issued by the Imperial Post Office himself, and that the stamps of the Foreign Post Office were to be added later. (These are know as "Combination Covers")
    b) Where an item is presented with the Foreign stamps are already applied, the postage stamps of China will not be added, but the strike of a c.d.s. belonging to the Office of dispatch was to be applied elsewhere to the front of the item, so as to mark its passage through that office. These are currently know as "Cash Covers".
(Imperial China History of the Posts 1897, pp 301-2)

A "Snell Cover" from Hankow to England.   April 29, 1897 Hankow Customs CDS. Frech 25c canceled by Shanghai French Post Office CDS on May 6, 1897.

On the back transit Shanghai Customs CDS of May 2, 1897, French ship LIGNE seal of May 8, 1897. Arrival Bristol CDS of June 14, 1897.

From Tientsin to US.   Tientsin Dollar Dater of Aug. 1, 1897. German 20pf stamp canceled by Shanghai German P.O. CDS on Aug. 13, 1897. Arrival Port Jefferson, N.Y. oval post mark of Sep. 6, 1897.

On the back transit Shanghai Dollar Dater of Aug. 5, 1897. Transit Vancouver post mark of Sep. 1, 1897.

From Peking to France.   Peking Dollar Dater of Aug. 14, 1897. French Post Office in China 25c stamp canceled by Shanghai French Post Office CDS on Aug. 20, 1897.

On the back transit Shanghai Dollar Dater of Aug. 19, 1897. Arrival Paris CDS of Sep. 27, 1897.

From Peking to U.S.   Peking Dollar Dater of Oct. 2, 1897. Japanese 5s Canceled by Shanghai I.J.P.O. CDS on Oct. 12, 1897. US Postage Due 5c (J41) was added at U.S. and canceled by Springfield CDS on Nov. 10, 1897. Handwriting "12½" and "CTMS T" mark and "U.S. Charge to Collect 5 cents" marks.

On the back San Francisco Cal. F.D. CDS of Nov. 5, 1897 and Springfield arrival mark of Nov. 10, 1897.

Letters sent before Oct. 1, 1897 affixed Japanese 5s postage according to Japanese office preferential rate. I.J.P.O. rate was changed to 10s from Oct. 1, 1897. The cover above was post-marked on Oct. 2, and still affixed with 5s Japanese stamp (short-paid by 5s or 12½ ctms). That should be the reason for the double penalty of the U.S. 5c (equal to Chinese Silver Dollar 10c or Japanese 10s, French 25ctms) Postage Due.