Prices are approximate because I don't know the real weight of the package but I suppose it.

Carol,

Great question.

The verb suppose means to assume, consider, believe or think something.  None of those meanings fit your sentence.  The verb estimate (to calculate approximately) is what you need.

There are also other problems with your sentence; here are my suggested changes:

These prices are approximate; because I don't know the actual weight of the package, I have estimated it.

I hope this helps.