Royal Mail handle the delivery of 99% of all letters under 350 grams, which means using an outside carrier isn’t an option as for the latter part of the journey, these services will also be affected.
A simple alternative method to avoid any further unnecessary inconvenience during the strikes would be to use electronic form of correspondence:
Sending invoices by email instead of post, its also a far more cost effective way of working – payments can also be made online by secure electronic payment methods avoiding any lost cheques in the post.
If you need to use a postal service, there are many independent couriers and delivery companies local to you, although it’s worth bearing in mind that most will be more expensive than your usual services.