Receiving post in BA from abroad

#1
I heard a rumour that if someone sends you a letter/small package to your Buenos Aires address from abroad (the UK & US in this case) then it can wind up sitting with customs for some time. Does anyone have any know-how/experience of receiving post from abroad?

I'm basically wondering how best, or if, to get some things from the UK/US to here: post + a frequent flyer card + a mobile phone sim I forget

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#2
I resend my mail from the States to Argentina. Letters only with an occasional replacement bank card.
I don't think that customs will be interested in your sim card.

Fedex always delivers on time. Sometimes packages are getting opened and resealed. Nothing is lost.
USPS Letter mail with $100 declared value takes about a month to get here.

Packages nobody has to sign for are getting lost.