Why do I have to pay for my new router in installments?

I recently ordered a new router and noticed on my bill that I am being billed in installments.  According to the customer service rep with whom I spoke (Ivy or Ivey), I have no other option but to pay in installments.  I was also told that if I send the payment in full it will be carried as a bill credit (and Verizon willl continue to bill me in installments).  Is this installment option designed to benefit the customer or Verizon?  Why can I not pay for the router in one payment?  I noticed on the bill that I will also pay additional monthly fees with the installment option.