de puerta a puerta ---> from one door to another (the kind of service DHL offers)
de puerta en puerta ---> one door after another (by knocking on doors all morning, like the Evangelical folk)
change puerta for whatever you need to mean locations. If you say "de pared a pared" (wall to wall), it describes a scope, or a span, or an expanse, or an extension: alfombra de pared a pared (wall to wall carpet) -never ever use en in these cases- atiborrado de piso a techo; corroído de proa a popa; sucio de la cabeza a los pies (or de pies a cabeza), etc.
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