Since you are located in San Francisco, the first question is whether your tenancy is regulated by the San Francisco Rent Stabilization and Arbitration Ordinance. Unless you are in housing that received a permit of occupancy in 1979 or later, or you reside in a single family home or condo, it is likely you have a regulated tenancy. If so, rental increases are governed by the Ordinance.
The second question is what did you agree to pay for in addition to your monthly rent at the commencement of your tenancy. For example, did you agree to pay for your utilities or other services supplied to the property?
If you agreed to pay for outside services such as garbage, the next question is whether there are other units in your building and are the services billed to the landlord under one account? If you reside in a multi-family building (say a 4 unit property) and the garbage services are billed under a single account, it is likely you have received a bill for your pro-rata share of the garbage bill for a particular period of time. For example, you have received a garbage bill for 1/4 of the entire invoice for a period of 3 to 4 months.
Assuming you agreed to pay for this service or utility, the invoice is real and has been or will be paid by the landlord, there is no unreasonable service fee tacked onto the bill and no excessive use charge that is the responsibility of another tenant that has been tagged on to you portion, it is probably a valid bill.
If you did not agree to pay for your garbage and your lease does not require you to pay for garbage, you should not have to pay for garbage seperately. And, under the Ordinance, your landlord cannot just decide that garbage is now your responsibility - even if s/he provides you 30-days notice of the change in terms of tenancy. That would be an illegal rent increase.
I hope this helps you understand the circumstances under which this would be a valid invoice and the questions to ask to determine whether it is a justified valid invoice you should pay.