Bel,
take a cab. Fare is around 330 - 400baht. So if the taxi drive say 400 baht for all (i.e. toll & the 50 baht service) take it. It's okay.
To Platinum Mall, take a cab. Should cost around 35 - 50 baht. The last time I drop off at Pratnum, cost me about 37 baht.
**At Samran, to catch a cab...best u cross over the other side. (i.e. when u leave the hotel, turn left...walk down, there is an overhead bridge to a 7 eleven...there also got pple sell sticky mango rice....easier to catch a cab) Be careful for the "tutu" drivers. There is a service staff there, a young man (more like 17/18 boy) he wears lipstick or whatever, but he is very++++ nice.