According to the article, that notation didn't come into use until the middle ages. So using IIII instead of IV was almost certainly acceptable in the context of transactions. Even if it wasn't, doing the conversion before addition/subtraction would be trivial (and the first step).

I believe some Roman inscriptions were found to use additive, i.e. III + I = IIII, VI + III = VIIII. Certainly I feel that nobody would look at IIII and be confused as to what it represents.

Lets say I am a farmer and I take eleven pigs to market. I start with XI pigs and sell three III. I know X = VV and I know V = IIIII so I expand XI = VVI = V IIIII I, then take way III sticks and am left with VIII. It is easier for simple primitive thing because the stick I represents a thing and the V represents 5 I's and X represents 2 V's. It is perhaps easier for someone like a farmer in Roman times to use than remembering arithmetic rules. It is more directly related to the physical world. At least that was my hypothesis.

They mention this in the article, and say there is very little evidence of this being common in ancient Rome.

Also it's not "really" hard. Think of subtractive notation as a way to write down what's really an int1 - int2 expression in normal roman numerals, with the out-of-order characters representing int2. To add these together, (A1, A2) + (B1, B2) = normalize(A1+B1, A2+B2) where those additions are a little easier. The normalize step is not very complicated; it basically says "if a letter is repeated on both sides eliminate one instance of it; if it is not but there are too many on one side, adjust the other side to compensate." The only nuance is that "too many" for the left hand side is "four" whereas "too many" for the right hand side is "two."