Seems no probs with that.
Those who live in the north may not be aware that down here the postal service treats all of Ireland as one unit; thus, if I post a letter from Cork to Coleraine, or Lifford to Strabane, it's local rate. I am aware that a letter coming the other way has to be treated as "international" postage.
Same for parcels. Now, as far as actual customs are concerned, it seems that so far anyway, the "protocol", the Common Travel Area, either or both, results in no issues for parcels posted south to north - not sure about the other way round.
It's when they travel from Ireland to Britain or vice versa that problems seem at arise. The only sure-fire way around it is for those of us who can, get anything ordered in Britain to be delivered to a friend or relative WITHIN Britain or the north, and for those who live in either, vice versa.
I now have routine that whenever a relative (who lives in the UK) visits me, anything I've ordered online from a British supplier appears too!