Una sugerencia.

Last time I went to the post office I purchased stamps. They were no longer of the regular 'lick-and-stick' kind; now, they are labeled 'sellos adhesivos de correos [de 0.58 €;]:

http://www.filatelissimo.com/la-expo-zaragoza-2008-tendra-su...

They still have the stamp features with little teeth, yet on the back, they are coated so you can peel them off the carrier foil and stick them directly onto the letter without moistening. I do not think that the context above refers to fiscal/notarial stamps and seals.

Of course the regular stamps are still available, too:
http://www.correos.es/comun/filatelia/2008/0450_08-seleccion...

Used for writing private contracts and the like, available at tobacconist's.

The stamp does not exist as a separate object, it's embossed in the paper, but the reference is usually to "timbres de estado"

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Note added at 37 mins (2008-04-04 11:43:55 GMT)
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I feel I should stress that we are talking about physical products for sale, or at least that's how I see it. So although timbre can indeed mean duty, excise etc, here we're talking about the physical bit, not the paying for it or of what is due.

If you google "timbre and Sello" a common term that appears is "timbre postal" which if you google as an image seems to be decorative, special collection stamps which are produced for certain occasions and anniversaries. This would make sense in the context of the text too.

Here's another suggestion.
Sello emitido por el Estado para algunos documentos, como pago al fisco en concepto de derechos.
tax stamp
excise stamp


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Note added at 5 hrs (2008-04-04 16:52:30 GMT)
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I would use seals for the first one and stamp for the second.

As opposed to the stamps we used to have to lick (Ugh!) and which often had insufficient glue, so they fell off!

Creo que Ametista y Dolores son las que andan más cerca.
Fiscal stamp/ duty stamp es la denominación del impuesto (en Argentina, se le dice 'timbrado') mientras que 'tax stamp' es la denominación del objeto con el que se realizaba el timbre/sellado de los docuemntos o correspondencia.



Stamp duty is a form of tax that is levied on documents. Historically, a physical stamp (a tax stamp) had to be attached to or impressed upon the document to denote that stamp duty had been paid before the document became legally effective. More modern versions of the tax no longer require a physical stamp.
http://en.wikipedia.org/wiki/Stamp_duty



While tax stamps were in use in Spain, there was mostly more than one stamp at a time. They differed for regular and non-standard cards, and there used to be another stamp for exportation.

This stamp was in use from 1904 to 1907.
The text is 'TIMBRE DEL ESTADO' / '10 CENTIMOS'.

http://www.unics.uni-hannover.de/rrzn/endebrock/stamps/st-e....

En el último link hay imágenes