This is what they are called in California, and it says here that there does not have to be a reason for cancellation, just follow the procedure within the time period and you are through.

The Act provides that a Doorstep Contract shall not be binding and conclusive if it is cancelled by the consumer.
Cancellation may be made within fifteen (15) days from the date of the private writing, or any longer period as may be stipulated therein. Cancellation may be made in any manner (by word of mouth, telephone, fax, or by submitting the ‘Cancellation Form’) provided the intention to cancel is made clear to the Door-to-Door seller or to the trader.

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Note added at 29 mins (2007-09-20 15:42:36 GMT)
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Auf Deutsch:
Nach der Richtlinie über Haustürgeschäfte hat ein Verbraucher grundsätzlich sieben Tage Zeit, um einen in einer Haustürsituation geschlossenen Vertrag zu widerrufen. Der Gewerbetreibende ist verpflichtet, den Verbraucher bei Vertragsabschluss schriftlich über sein Widerrufsrecht zu belehren.

This is in the same vein....just a small variation: "door-to-door" :-)

Consumers' right to cancel door-to-door sales contracts
A consumer's right to cancel certain contracts is referred to as his "right to rescind" that contract.

[PDF]
Door-to-Door Trading Act 1991—Form 1
File Format: PDF/Adobe Acrobat
Notice of consumer’s right to cancel door-to-door contract. (This notice must be read aloud in full and then given to the consumer at ...