Question about Maul electric letter opener

I've been using it for 3 months and open all my letters with it (10 a day) and have only once had a letter that was too thick and there were an estimated 20 sheets inside.

We open up to 40 envelopes a day and have to change the battery every 5 weeks. The machine is great, opens the letter in 2 seconds.
It is always thin at the top of the envelope, it only needs 3mm to open.