The strikes have already cost the sector hundreds of thousands of pounds.
Charities that rely heavily on Christmas appeals, those currently running campaigns to recruit new committed givers, and those appealing for cash donations have been badly hit as consumer confidence in the postal service has plummeted, and some supporters have decided not to send cheques in the post.
The Charity Christmas Cards Council, known as 4C, has lost £250,000, while the Society for the Protection of Animals Abroad estimates that it has lost £60,000 as a direct result of the strike.
The charities now fear the strikes could reverberate throughout the sector in the longer term.
"People could continue to be cautious about sending donations in the post for some time yet. Although a truce has been reached, the strikes could start up again if workers feel that the deal isn't what they first expected," said Caroline Pons, director of fundraising and publicity at the Society for the Protection of Animals Abroad. "We have lost £60,000 so far, and will also lose out in the long term because we have missed out on recruiting some committed givers - it is impossible to estimate the value of that."
4C, which was founded by charities for charities in 1966 with the aim of promoting charity Christmas cards, also foresees a long-term impact.
"The future is not looking rosy and the difficulties with the Royal Mail have been the last straw," said Neville Bass, 4C's chief executive.
The wildcat strikes, which involved about 20,000 workers across Coventry, Warrington, Hatfield, Chelmsford and parts of London, led to a huge backlog of letters and thousands of post boxes being sealed.
The action was called off on 3 November after the parties agreed to take the dispute to ACAS, but Royal Mail has admitted that the backlog could take between two and three weeks to clear.