There are millions of different items, you can hardly expect PO staff to know every rule for every possible item that someone might send.

Weekends are not included in their times because Saturday delivery is a special service & there are no deliveries at all on Sundays.
I have had a refund for one sent RMSD which was refused air cargo & so went by road & took several days.

PF48 is the only PF service you can use.

RM state 1st class as the minimum, but as that only has £50 cover & only goes to a med size parcel it's not practical,
RMSD, guaranteed next day, has £500 cover so is a good option if you can get inside the size limits.

Large parcels must use PF, & airguns on PF must use PF48 only, but make sure the PO does not confuse that with PF48 large which is for really big stuff.