Etiquette Snob here…
Ok this is a toughy… as it is never appropriate to mention Gifts at all from a traditonal etiquette point of view.
As you said, the best bet is to (a) have a small registry, and (b) folks spreading the word that you’ll be heading back to Canada after the Wedding
Those elements alone, and hopefully folks will take the hint and mostly send you Cash Gifts or Gift Cards (double check all your Gift Cards… because altho for example WalMart is in both Countries… Bed Bath & Beyond etc… it doesn’t necessarily mean that the cards will work in both countries. If you get US Cards you’ll want to use them before you leave the USA to head back to Canada, or see if you can’t order off the US Websites for US Delivery. Not sure if that is an option with all Interntional Retailers… sometimes they want you to only use the company in your country)
Both your Canadian & American Guests can choose to send any presents directly to you in Canada if you have a contact address for them there… especially as so many people order off the Internet these days.
Where are your RSVPs destined for ? That is usually the address that folks use when forwarding gifts.
(Infact proper Etiquette is to send the Gift BEFORE the actual Wedding takes place… altho not everyone knows this)
You might want to consider a Canada Post Office Box or UPS Store Post Box… both would be in a position to HOLD items for you if you make prior arrangements (UPS would probably be the easiest of the two)
As far as Customs goes…
As Canadians… when travelling into the US for the Wedding, if they have a gift in-hand they will have to of course declare to US Customs… they are allowed to bring into the USA a gift worth $ 100 (per person, but not combinable… so a couple cannot claim a $ 200 item). And they must be staying in the USA for 72 hours or more to qualify for this exemption.
US Customs & Border Protection = https://help.cbp.gov/app/answers/detail/a_id/975/~/visitor's-gift-exemptions
I think a lot of Travellers get around this, by choosing to buy their Wedding Gifts when they arrive in the Destination
Lol, then it is your problem…
As a Canadian Citizen…
You’ll be able to claim a personal exemption up to $ 800 Cdn once you’ve been out of the country more than 48 Hours
Canadian Border Services Agency = http://www.cbsa-asfc.gc.ca/publications/pub/bsf5056-eng.html#s2x8
If your New Hubby is travelling back to Canada with you, as a US Citizen visiting Canada he’ll be able to claim only $ 60 Cdn as a Gift allowance
CBSA = http://www.cbsa-asfc.gc.ca/publications/pub/bsf5082-eng.html#s2x1
Honestly I would say your best bet in this situation is to…
(a) Utilize a Post Office Box for your Mail / Parcel Delivery during the Wedding Plans
(b) Consider using a Canadian Store for your Wedding Registry (such as The Bay)
(c) Get the word out that you will be in transit the first part of your Marriage, so there will be cross-border “moving issues” (don’t mention gifts at all)
(d) If you have a Wedding Website… dedicate a section to “After the Wedding”… and talk about your plans for relocation / moving
(e) Engage your Friends & Family Members in spreading the word… coming from them it isn’t seen as being overly focused on the material aspects / gift grabby
Hope this helps,