If you want to change your shipping type on Mandarake to one you can't select, you need to use the contact form to contact the store, prior to paying (you can close out the order at the payment stage). I have done this once to get something sent by air instead of SAL (I also put it in the order form, then read the instructions that say you need to message the store with the contact page!).
The restrictions for SAL are that the box width+height+depth cannot be more than 90cm, and the max weight is 2kg. Mandarake provide the item sizes and weights so you should be able to calculate if your stuff fits in a SAL box or not. With Sahara, the checkout will do this calculation for you.
No idea what would happen if your shipping request gets denied, if they'll permit cancellation or not.
For me, I've always had the SAL option available in the checkout for Mandarake (am in the UK).