The key to ensuring your mail makes the most of the space available.
Postcards
When using our designer you'll notice there are several markings and highlighted areas. For example see the below image which is the back of a 4X6 Postcard.
As we print your address and indicia permit stamp directly onto your postcard there are certain areas that we need to be left clear in order for it to stay mailable. The striped section are clear zones. The red border are the Bleed Margins. The green dotted line are the safe zones. These marks are not printed on the design and are just used to assist you in making the best design possible. 
The 4X6 Postcard is quite small and so the Clear zone is quite large. If you want more space then we can recommend our 6X9 or 6X11 products. As they are physically larger there is more space available to include your designs.
Trim marks
When printing we print on a larger paper and then cut it down to size. We use these guide lines to aim where to cut.
Bleed Margins
This area is cut off. Make sure background colour overflows into this margin for an edge to edge design.
If part of your design is placed within the Bleed Margins it has a chance to be cut off of your letter or postcard. We recommend ensuring everything is within the safe zone.
Safe Zone
Keep important text and logos inside the safe zone. This is because printers can sometimes print with a minor skew.
Clear Zone
We need to make sure addresses and barcodes can be machine readable by keeping these areas clear.
Letters
For letters there are also two clear zones. One for the address block and another at the bottom left for a integrity QR Code. The integrity QR code is an amazing feature to ensure your pages are printed in the correct order and enveloped into the correct envelope. We use this to assure our customers that your post will not get mixed with another.
We recommend leaving both of these sections clear. Or else it may fail our quality control checks.
Additionally as letters are folded into an envelope, it can be handy to know where the letter will fold. That's why we've provided two grey dashed fold lines on the letter design. This will not be printed on the design.
If your design has content within the clear zones, we can overlay with a white overlay. To do this yourself, go to your Mail Piece Settings then select the "Place white overlay" option.