I've been using Taxslayer the last few years, but found myself in need of an IRA recharacterization, and found out that they don't support attaching the supporting statement.
I'm a sicko that likes trying out different tax softwares each year (comparing their results to find things that were missed in one or the other until I get a consensus on my tax return), so I also ran through TurboTax, HRBlock, and TaxAct. Results? Couldn't find any way to attach supporting statements for TaxSlayer nor TaxAct (I could barely get TaxAct to recognize my various IRA issues). TurboTax and HR Block both recognized my recharacterization and prompted me to provide a statement. I'll also note: HRBlock was bass-ackwards about how it identified my IRA situation, but eventually got to where it needed to be. TurboTax was much cleaner about it, but at almost 50% more cost, no thanks.
As noted by others, simplest solution is to print it out and mail it in? I did take a look at Form 8453, but it seems to have a fairly specific list of things that it expects to be attached.