Some of the items available from the shop are polyurethane resin casts (or very occasionally, Jesmonite) as opposed to direct 3D prints.
The upside of this is that it's much more economical than directy selling 3D prints - where the design allows casting - but there is some prep involved in resin casts sometimes.
The main issue being air bubbles:
Large, problematic airbubbles will either be fixed before the item is sent to you, or just wont pass quality control, but it's possible your casting will arrive some small bubbles that will need filling.
Step One: wash the thing! Mould release helps to get the casting out of the silicone mould, but that will also stop paint, putty and filler sticking to the casting - so do this before attempting to fill any holes with warm, soapy water.
Then, identify any holes and split them in to two camps in your head - ones on a surface that can be sanded easily (i.e. without harming any fine detail) and ones that aren't.
For holes on a nice, easy surface just slap some filler or modelling putty in and rub it flat. One's it's dry just run a file over it, then some wet and dry paper.
For trickier holes - take the same approach but before the putty dries, take some time to sculpt it into the right shape, nice and smoothly. Once dry, just take a small file and tidy it up to smooth it.
Lastly, the nature of single sided moulds can mean the bottom of the cast might be a touch uneven, so give it a blast over some sandpaper to even it out.