The ATMs, like the "traditional" stamps, offer many study possibilities. 
In this article we'll outline a new aspect, the postal tax documents franked with ATMs.

Generically, the tax or postal rate is the amount that the user of the mail must pay for the transport and distribution. 
In some cases the post office's employees fill up forms, many of them no more in use, that justify the payment of these special taxes; the value appears in form of stamps and/or ATMs.

In all the cases this documents are for internal post office use, often with peculiar postal marks. They keep in the post offices archives and, therefore, very rarely can arrive at hands of the collectors.
 

	Model F.I. - Insufficient franking If the shipment is not correctly franked, when arriving at the destination the postman collect to the addressee the double of the franking insufficiency and gives the shipment.  With the money, and already in the post office, he acquires a stamp to frank the form as an internal voucher.  This system is already in disuse.

The same form F.I. was reprinted many times during the years with a few variations. We show here some examples ...
 

Model M-18 - Insufficient franking Similar to the previous one, this form announce that the shipment is stopped in the post office because the franking is insufficient.  This note can arrive at the addressee once arrived the shipment, but also at the sender before leaving.  These documents usually include two postmarks with date, that give information about the period during which the letter has been stopped in the post office; the date of the note and the date when the corresponding rate became paid.  In theory a shipment could not be retained in origin, but we see as this form allows that the post office decides where the rate will be paid.

There exists also many reprinting of the same form ...
 

	Model M-4 - Notice of arrival In the case of return of reimbursements, postal parcels or registered printed matter, a new rate equivalent to the value of the franking was collected again from the sender. The from include the stamp with the amount of the rate to justify and to demonstrate the payment.

	Model M. 15 This form is used for the post offices internal registered shipments between departments, although here it's used to collect the reimbursements return, as the previous case.  This use of forms for different uses occurs in many occasions, some times because the postal employees don't have the correct form or some times because it's more easy ...

	Model F.D. 1 - Franking at destination Some companies offer the franking at destination service (F. D.).  This is the invoice ; a copy with the stamp for the total amount is for post office archives.  In case that the payment does not become cash, it can be reduced of the guarantee previously established. Normally the stamps are in the back of the document.