I don't understand why you have I as a delimeter... it may be that that's interfering w/ the ID option for cart links and keeping it from adding.

What's the delimiter for? In this scenario:

/cart/add/e-p43_q1_a1o12345I23666I58886?destination=cart

a=1 (the attribute id from the uc_product_options and uc_attributes_options tables)
o=12345I23666I58886 (supposed to be the option id from the same table(s))

The main issue is that, you're not using a database value if your value for "o" is a string and not a number. It appears that in the tables mentioned above, the Type for oid is mediumint(9)

So what's happening (probably) is the Cart Links is looking for an integer, but you're not supplying one, so it's only giving back the first 9 digits of your option ID. But, since your option ID probably doesn't correspond with any value in the database, it's most likely not returning anything at all.

What you'll need to do is look in your database, or in the attribute/option editing screens, and get those values, which you'll then substitute in your a#o# formula.

So then once you've created your Invoice attribute, and its three options:

a1o1 = Invoice 1
a1o2 = Invoice 2
a103 = Invoice 3

So your link would look like this:

/cart/add/e-p43_q1_a1o2?destination=cart

(keep in mind the values will probably be different in the database - you'll need to remember the IDs of each attribute and combo, either from the Edit Attribute screen or from the uc_attributes_options table)

Or if there are additional values you need to grab, you're better off (for now) writing a Contrib that grabs them from the query string like this:

/cart/add/e-p43_q1_a1o2?destination=cart&invoice=foo&option=bar or something ... still unsure of what you need the I delimiter for but I trust you'll be able to figure it out