can I tax a car at the post office with std PIP

I'm confused by information. I receive std mobility PIP and have just bought a second hand car as I had to hand back my motability car last Monday. I have all the paperwork and people are telling me I can tax it at the post office but I've seen on the internet that as its 50% discount I have to post it to the DVLA.
I have to have it taxed by Tuesday 2nd August when we pick it up.

I have recently bought my motability vehicle having been awarded standard rate mobility PIP. When I went to the Post Office there were two problems -

Firstly, I had my award letter but have to ask DWP for a special form in order to claim reduced tax.

Secondly, her till/computer had no facility for issuing half price tax. She could do free or full price!

As my tax was only £30 for the year, I paid this and will sort everything out with DWP and DVLA before it is due again!

what an effort, we went to the post office and thinking we had everything we needed we were in for a shock. The post office said I needed a certificate from the DWP and I said its for 50% reduction in car tax and he said he'd not heard of anything so I had no choice but to pay for a full 6 month tax so we can collect the car on Tuesday.
All this extra walking has caused major setbacks in pain levels and loss of mobility.
I'm hoping that once I get the V5 I can send the list of documents to the DVLA and get a refund on the full 6 month tax.
Do I have to do the same to renew tax every year.
This is a new world as for the last 15 years I have had a mobility car.

Going round in circles, can anyone tell me if I have to change the Taxation Class on the V10 or V5C. Tried asking DVLA but getting conflicting information, I have even involved my MP!