If it's parcelforce, either wait until they post you an invoice or call up their helpline with tracking number and ask nicely if you can pay over the phone without invoice number.
If you give them a call and they have your parcel, they'll probably let you pay over the phone...although I have no experience of this since I work about 200m away from them.
You can call up Parcelforce ahead of the invoice arriving to pay over the phone.
I had to go and pick the parcel up from somewhere towards Croydon as they couldnt drop the wheels off, nor did they leave a note. Think it was about 35 quid for mine.
My Light Bicycle rim is "Depot, Awaiting payment of customs charges"
Any idea how I'm supposed to
a) find the depot
b) pay the charge to get the fucking thing delivered