Writing off part of an invoice

I have purchased a new printer and, at the same time, I have also purchased an additional set of toners for this printer. The accountant informs me that I can/may write off the printer over 3 years. However, I may book the toners in full as office supplies. But how do I split this up in Dexter, knowing that both items are stated on the same invoice? So, write off part of it over 3 years and book part of it in full as expenses.