Paid-for marketing material is not being delivered due to Royal Mail's backlog, it is alleged
The Bristol Mail Centre often makes no attempt to deliver paid-for marketing campaigns, according to Ben Watts of the Communications Workers Union (CWU).
Mr Watts said staff had reported being uncomfortable with the situation.
Royal Mail said it was not policy and staff are often reminded that all mail is of "equal importance".
Door-to-door mail is a service offered by Royal Mail to businesses, charities and other organisations hoping to reach new customers or donors.
They are charged a fee to have a leaflet or letter delivered to every home in a certain post code over a particular time frame.
In an open letter to the Bristol Mail Centre's management this week, Mr Watts said he had received "several complaints" from staff about the treatment of door-to-door mail at the facility.
Industrial action and a moratorium on overtime had contributed to the backlog, Communications Workers Union branch secretary Ben Watts said
Mr Watts, who is secretary of the Bristol branch of the CMU, alleged that parcels were being prioritised, and leaflets were deemed "out-of-date" if the delivery period the client had specified was missed, they were then thrown away.
"I am sure the customers who have paid for (door-to-door mail) will be disgusted," he said.
Mr Watts added: "This hit a real nerve when one of the door-to-door leaflets being binned was a charity asking for donations."
Speaking to BBC West, Mr Watts said: "Maybe the public don't like it, and the posties don't like delivering it, but door-to-door mail is a really important source of revenue for Royal Mail."
He added: "Are we letting the companies know we are not delivering the door-to-door mail they've paid a substantial amount of money for?"
Mail is being stored outside the Bristol Mail Centre
Mr Watts said the backlog was not only the result of recent industrial action, but was also down to a moratorium on overtime imposed since August and only partially lifted this week.
He said before Christmas, many postal workers would be doing between 10 and 20 hours of overtime per week.
Mr Watts said the moratorium had been a "strike breaking tactic" as workers would be less likely to strike if they were unable to make up lost wages through overtime.
It is understood Royal Mail does not have a policy of binning undelivered items of post.
A fox is spotted climbing on post at the Bristol Mail Centre
A spokesman said: "Every item of mail is important to us. Royal Mail does not operate a policy of prioritising parcels.
"We regularly remind colleagues that the delivery, collection and processing of letters and parcels should be treated with equal importance."
He added: "Due to the amount of space they take up, parcels can restrict the movement of both people and mail in our offices leading to health and safety issues and delays to other mail.
"At particularly busy times, such as in the recovery days after a strike, we may occasionally at a local level clear parcels to free up space and address health and safety concerns so that we can keep all mail - including letters - moving efficiently through our network."