ASTM standard F2907-15 lays out the requirements for selling wrap and ring sling-style carriers.

What the standard says, in a very small nutshell:

• All wraps and ring slings must pass testing using the manufacturer's directions before they can be sold. The testing is looking to make sure that the fabrics used are safe, and that the carrier can be used safely as it is designed.
• The standard specifies that testing must be redone whenever there is a "material change" in the design, or annually, whichever comes first.

What does that mean?

As an example, if I make a WCRS out of a Didymos wrap, it may not perform the same as a WCRS made from a Girasol wrap, because the two wraps are from different manufacturers, and I would need to sew and test both. Each testing cycle (which must be repeated annually) costs more than $500 per item. It simply isn't possible for me to test a sling from every wrap manufacturer. In fact, I need to sew a minimum of 25 slings from a given manufacturer just to break even on testing costs, and as a business owner, I need to take care of my bottom line and make sure I'm not just breaking even, but making a profit to provide for my family.

"Material change" also means that once a wrap has been used, there's no way to know how the fabric will hold up in testing (since it might have been exposed to smoke, or stored for years, or improperly washed, any of which could damage the weave and lead to a failure), and while I suppose it's possible to send in half of a wrap as a WCRS for testing and sew the other half once it passes, that would make for a very expensive sling! (The testing done is a destructive test, and while fabric can be returned from the lab for an extra fee, it can't be used as a carrier anymore.)

These factors add up to the sad reality that custom WCRS are just not practical anymore without cooperation between the converter and the wrap manufacturer.

What I can still sew:

I have completed testing on instock French twill, linen, water mesh, and my Tekhni Bios exclusive. Because of the latter, I can still convert new Tekhni Repreve-blend wraps. I have also worked with Meeyoo on doing WCRS with Girasol standard-weight broken twill weave wraps (not the thick twill or diamond weaves), with a gathered shoulder, so they're okay, too (but only with the gathered shoulder). These are the only non-instock WCRS I can legally continue to sew.

What I can't sew:

I am not be able to do conversions on anything I haven't tested, and anything used is also a no-go. This means: no wraps other than those listed above; shoulder redos even on brand-new slings (since I haven't tested the fabric from that manufacturer, unless it's a Tekhni Repreve or Girasol WCRS); and other customer-sent fabrics, such as silk or handwoven pieces.

What this means for all sling sewers, regardless of their size:

The sling market has changed drastically, in that those one-of-a-kind conversions are no longer be allowed, nor the legacy-wrap type conversions. I'm sure there will be unscrupulous (or unknowing) people who will continue to do them, but they will be in violation of federal laws, and I will not be among them. And sending your wrap to Canada will also not be a workaround, since any product sold into the US has to comply, so your Canadian converter can't legally send it back to you. (Plus Canada generally adopts US guidelines on product testing, so the regulations will be similar, if not the same, in Canada.)

Some people who sew carriers are under the impression that your business has to be a certain size for the regulations to apply to them. That is not true. The regulations apply to anyone who accepts money to sew a carrier -- that's what defines a manufacturer. Whether you make 5000 slings a month or 5, when someone pays you to do it, you're a manufacturer, and your carriers are legally required to comply with the regulations. That's not just testing; it's also the rules that went into effect in 2011 for labeling and tracking. You can read more about those on the BCIA's website, and I recommend contacting them for compliance help if you're just starting out. If you're already making non-compliant carriers, you need to stop ASAP until you're in compliance. Continuing to sell puts you in very real legal jeopardy. It's tempting to assume you will just slip under the radar if you're small enough, but that hasn't been the case in the past, and is unlikely to work for you in the future, either. This is not something you can ignore.

A note on abbreviations you might see:

I am a member of the BCIA, the Baby Carrier Industry Alliance. (Actually I was a founding board member, but now I'm just a regular member.) The BCIA is essentially an industry club. While there are BCIA members who volunteer to work with the CPSC (Consumer Product Safety Commission) on the ASTM (American Society of Testing and Materials, though now they just use ASTM because it's international) sling standard, the BCIA does not set or have other undue influence on the standard's creation.

They help members reach compliance with the standards and the CPSIA (above), but there is no "BCIA certification" or "BCIA compliance". Either a business complies with the standards and rules or they don't; the BCIA doesn't judge that or certify its members. The phrase "BCIA Compliant" in a seller's listing is a huge red flag that they are probably not following the rules, since they either don't understand or are ignorant of what they actually need to do to comply.