These are upgrades to your current account that will allow you to permanently spawn with a specific knife. Below is a picture of all the current permanent knifes you can buy. You may only have one perma knife enabled at a time, you may switch which knife you have by going to the very bottom of your credit shop and enabling a different knife, which will auto disable your old one.
Basic KnifeĀ will add the basic knife as a perma weapon to your account for 300 credits. These are also purchasable from the BMD.
Skinned Knife'sĀ will add whatever skinned knife you buy for 800 credits as a perma weapon. You can not get a skinned knife any other way, and they will do more damage than a basic knife, but grant no advantage over any other skinned knife.