The problem with tearing letters is one of mocheik – erasure. On the Biblical level, only a constructive erasure – like preparation to write something else in – is forbidden. On the Rabbinic level, even non-constructive erasure is forbidden. The case of opening a package would be a Rabbinic prohibition at worst.
Additionally, in this case, one has no intent to tear letters. One is concerned with the contents of the package, not with tearing letters. Under normal circumstances, this would remove any prohibition. However, if a prohibited act is the inevitable result of some other act, that act becomes prohibited – a category of prohibition called ‘psik reisha’. The question is, what’s considered ‘inevitable’? If I can open that package, and one time out of ten not tear any letters, I think we can reasonably say that the result of tearing is not inevitable. But what about 1 in 100? 1 in 1000? 1 in 1,000,000? Etc. It there some kind of threshold after which something is treated as inevitable even if there is a miniscule chance of it not happening? If so, what is that threshold?
In the case of the packages, I think that one is within the threshold. There are other situations where tearing is truly inevitable – popping pills out of a labeled pod, for example (I take one where I have to open it by other means on Shabbat). There, opening it carefully won’t do the trick.
Let’s summarize. The question is about opening some type of wrapper where tearing letters is either inevitable or close to inevitable. We start with the proper question: Why should it be assur?
By destroying words, you are erasing. However:
1) There’s no constructive purpose to the erasure (making it a drabanan)
2) This is not the normal mode of erasure (Chavot Yair uses a similar svara to explain why one can eat foods with letters baked into them, though it’s possible to distinguish)
3) Erasure is not absolutely inevitable (especially if you throw in R’ Shlomo Zalman Aurbach’s psak that the problem is with destroying letters, not separating between letters)
4) The one doing the tearing doesn’t care about the torn results (so even if it’s a psik reisha, it’s ‘lo ichpat lei’, which may have the same status as lo nicha lei)
Where does that leave us? There is a definite case to be made that it should be prohibited if tearing is truly inevitable (perhaps one day I’ll get around to experimenting with a million Laffy Taffys to determine the inevitability of tearing letters). Nevertheless, there are definite reasons to be meikil (at the very least, it’s a psik reisha on a drabanan, on which the Shulchan Arukh brings meikilin). Would I be careful not to tear letters? Probably. Would I teach this as halacha? Probably not to people whose attitude to the Laws of Shabbat is, let’s say, underappreciative. There’s so much more to learn and understand first.