I have bought ALL my shrimp through the post, and majority from ebay and not yet had any problems at all.
As JayC states you have to almost assume the shrimps in the pictures are stock photos off the internet, though the ones on this advert are at least all the photos are taken from the same tank so chances are they are likely to be from the sellers tank.
Make sure they are sending them 'next day by 1pm (TM) signed for Royal Mail' which is usually around £8-£10 cost, this advert will but you have to choose that postage at £8.50, so only £2 extra over the standard 1st class.
Probably the only area where we are luckier than the large countries like USA and Australia is that because we are so small it is easy to order these through the post and get them next day! There are a lot of people selling shrimps through ebay as well over here. Here is my usual search:
https://www.ebay.co.uk/sch/Pet-Supplies/1281/i.html?_from=R40&_nkw=shrimp&_sop=12
The ones you have picked out may of coarse be sold by June , but there are always plenty, but as per JayC be cautious over the photos as they may not be theirs, OR they may be enhanced. You also MAY loose a couple due to the stress etc (being packed away, thrown around etc), but you are not as likely too as you know what you are doing - I used to assume I would loose 2 of 10 and then was delighted if they all survived.................
Also, by June you may not need a heater pack (hopefully) but if it is cold , as now, make sure they will be sending a heater pack (some people will charge extra for that).
I have read several places (and believe it to be true from what I have seen), that baby shrimp grow 50% with each molt at the start, and as they do grow so fast they must molt very often!
Simon