Registered letter sent back because Postnet didn't put slip on my box

I was sent a Registered letter from overseas but it was sent back because Postnet said they only keep it for 3 months. However I was in Postnet 1.5 months before and there was no collection slip for this item. As we seldom use normal mail anymore I can't afford to keep going regularly anymore to check my box. To help we customers, I asked for a SMS to be sent to me when a registered letter is received but I was told "no" they don't offer that service. This would provide a safety net if they don't put a note in my box that there was a registered item waiting for me. How much does a SMS cost to provide good service.