Multiple carriers for same order over charging

[followme]

I’m using PrestaShop 1.6 (default theme) and the standard PrestaShop cart.  and its the carriers postage posting and post section , i am not using any module its all in std for 

 

Multiple carriers for same order over charging

2 couriers
            

              All postal prices per items / products we charge by weight only to deliver in the UK

2 couriers and the system works perfect in each separate group

•      a> Letter post   ( small Items cheap to post )  "weight to correct postage price "
•      b> Parcel Post  ( large items cost more )  "weight to correct postage price "

so if

All products are in A> group "Letter post" all postage works and checks out perfect  to weight value
All products are in B> group  "Parcel Post" all postage works and checks out perfect to weight value

and if

2X All products are in B> group "Parcel Post"priced from weight say   £3.20 ( 250g x 2 = 500g  weight )  = £3.20 postage  in total price to post

1X  products are in A> group  "Letter post"priced from weight  £1.25  ( 20g ) in total price to post normally by its self but

when ordered together

500g +220g  both A> B> set at = £3.20  for default carrier because its under 1,000g  weight

This is not happening and the customer is getting charged  £3.20  + £1.25 -=£4.45 ( and NOT just 1,000g at £3.20  this is an example and we have a lot of small low tickets items and now daily getting dropped carts because of this issue

It charges correct for each group carriers BUT THEN ADDS both  A. and B> together NOT the TOTAL WEIGHT as it should charge as a combination

I did set up the default weight carrier to be B> group  "Parcel Post"  just for this problem but not doing this either

So now its over changing postal by adding them both together when NOT combining the weight and defaulting to the price of the weight  to postal B> prices

please help .........

:-(  Ste