Poly mailing or postal bags are an ideal way to secure your item to send in the post or via a courier.
Sizes vary and its always best to allow a 2% margin of error in any sizes quoted. Mailing bags are strong and usualy have a sturdy self adhesive strip along the flap to easily seal the package. You can buy them with pre-printed address boxes but if you purchase without this option then a good rollerball or biro pen will do the job when writing the address on or indeed a sticky label is ideal! It is always a good idea to write the address on the bag before you add the item for posting as its a flat surface to write on.
Protecting the item inside is always a good idea and these poly bags allow for this. Wrap your item for posting in bubble wrap and buy a poly bag that is that little bit larger to allow for the extra size with the bubble wrap.
Remember that ROYAL MAIL sizing in proportion rules mean it may be better to lay your parcel out flat instead of rolling it up (where this option is allowed) and keep it to an A4 size as this is then a large letter, rolling it up will mean its thickness will probably put it in a packet size and the posting cost will increase.
Buy a few bags more than you think you need, some times we make mistakes and only realise after we seal the bag, but if its a simple thing like leaving out a small item or messing up the address, don't rip oppen the bag but put it inside another bag add the item or re write the address. The first bag if ripped open is wasted anyway and adding a second bag just makes it more secure and its the quickest and simplest way to sort out said faux pas.
So which bag to choose, there are so many options and colours, decide on what it is you want. If you are sending a birthday present then a bright coloured bag says that little bit more but all in all the poly bags make posting items simple and cost effective.