I saw the sign pictured above over the Christmas break (hi Yarragon!), and realised something: I didn't know if there was any underlying principle to when a word is spelled with -able or -ible on the end. It was time to find out.
The Macquarie Dictionary notes that while collectable is the dominant spelling, collectible is also acceptable. It also offers the following note on the variant spellings:
The suffix -able is the form used with words of non-Latin origin (as in knowable, readable but Latin words borrowed into English come with either -able or -ible depending on the stem of the root word in Latin. The two are interchangeable in some derivatives, especially those which have been formed in modern English.
Given that almost no-one speaks Latin these days, that's not necessarily a good basis for distinguishing which one to use. The fact that exceptions are becoming increasingly common reminds us that etymology is an interesting hobby, but isn't always a helpful guide to meaning or spelling.
The most useful rule you can deduce from this is that if you're unsure which spelling to use, -able is more likely to be correct. But if you're not sure, check in a dictionary (or pick an alternative word which you can spell). Accuracy matters.
Lifehacker's Mind Your Language column offers advice on improving your writing.