Suha should buy the 50 envelopes from Letter2send.
Explanation:
Step 1:
At Letters2send, a pack of 25 envelopes costs £3.49. We need to determine how many packs will have 150 envelopes.
The number of packs =

So 6 packs of 25 envelopes will have 150 packs.
The cost for 150 envelopes =

So at Letters2send, 150 envelopes cost £20.94.
Step 2:
At the stationery word, if you buy 2 packs of 10 envelopes, you get a pack of 10 envelopes for free. So 3 packs cost £4.20. You get 30 envelopes for £4.20.
The number of 30 envelope packs =

The cost for 150 envelopes =

So in the stationery world, 150 envelopes cost £21.
Step 3:
150 envelopes from Letters2send are cheaper than the stationery world so Suha should buy the envelopes from Letters2send.